Anticipatory wheel-running in behaviorally anosmic rats

Rats rendered behaviorally anosmic by the intranasal infusion of zinc sulfate were placed on a single daily meal, 2 hr in duration. Anosmic rats and controls showed a similar degree of wheel-running anticipatory to the meal. It was concluded that the stimulus for entrainment of meal-associated rhythmicity is not olfaction and that other characteristics of food intake need to be systematically investigated.     

Periodic water availability is not a potent zeitgeber for entrainment of circadian locomotor rhythms in rats

Entrainment of tilt-cage measured circadian activity rhythms to a 2 hr daily period of water availability was assessed in rats fed either an oil-chow mash, which rats readily consume in the absence of water, or dry pellet chow, which rats less readily consume in the absence of water. Four of 6 pellet fed rats but only 2 of 14 mash fed rats exhibited entrainment to water access. Periodic water availability, unlike periodic food availability, thus does not appear to be a potent zeitgeber for circadian activity rhythms in the rat. The entrainment that does occur in some rats may result from shifting of food intake to coincide with the availability of water.     

Failure of periodic presentation of palatable diet to entrain feeding,drinking and activity rhythms under constant conditions

Feeding, drinking and activity of two groups of male laboratory rats were studied across three experimental stages: LD 12:12; LL; and LD 12:12. In the first group (Control), ordinary diet was availabel ad lib for the entire experiment. In the second group (Experimental), palatable diet was available for the first 12 hr of each day and ordinary diet for the remainder of the day in stages 2 and 3. It was found that in stage 1 all behaviors were highly nocturnal. The nocturnality of feeding was achieved by the ingestion of more meals in the D phase as compared to the L phase. Dark ingested meals were not significantly larger than meals in the L phase. In stage 2, circadian rhythms in all three variables were damped and meals became evenly spaced across the day in both groups. In the Control group, meal sizes were also of the same magnitude across the day. However, periodic presentation of palatable diet resulted in larger meals and, therefore, the ingestion of 60% of total daily food intake during the 12 hr period when palatable food was available. Total daily food intake in LL did not change compared to stage 1 and did not differ between the groups. The size and number of significant meal size/post-meal interval correlations increased over the duration of the experiment but remained relatively small. In stage 2, water intake and activity were similarly evenly distributed over the 24 hr period in both groups and were not influenced by periodic presentation of palatable diet. No marked decreases in total daily water intake, but a substantial decrease in daily activity, were found. Under LL, daily body weight increments in both groups were maintained, but subtle changes were observed. In stage 3 when the LD cycle was reinstated, both groups ate a greater number of meals in the D phase despite the availability of palatable food in the L phase for the Experimental group. These animals ate fewer but larger meals of palatable diet than ordinary food resulting in 50% of their daily intake being ingested during the L phase. Activity and water intake returned to stage 1 levels and the distributions of these behaviors were similar to those found in stage 1 in both groups. It was concluded that palatable diet influenced meal size only and that the timing of meals depended on the illumination conditions only. Further, periodic presentation of palatable diet failed to entrain the damped rhythms of feeding, drinking and activity in LL. These results do not rule out the possibility that such an entraining effect of periodic presentation of palatable diet might be found in conditions of DD.     

Entrainment by a palatable meal induces food-anticipatory activity and c-Fos expression in reward-related areas of the brain

Rats maintained under restricted feeding schedules (RFS) develop food-anticipatory activity and entrainment of physiological parameters. Food entrainment is independent of the suprachiasmatic nucleus and depends on food-entrainable oscillators (FEO). Restricted feeding schedules lead animals toward a catabolic state and to increase their food driven motivation, suggesting that in this process metabolic- and reward-related mechanisms are implicated. This study explored if motivation driven by a palatable meal is sufficient to produce food-entrainment. To address this question, we evaluated whether daily fixed access to a highly palatable meal entrained (PME) locomotor activity, serum glucose and free fatty acids concentrations in rats maintained without food deprivation. The entrained response of PME rats was compared with rats entrained to RFS. In a second experiment, we used c-Fos-IR to identify structures in the central nervous system involved with PME. Rats showed anticipatory activity to a daily palatable meal, with a lower intensity than rats entrained to RFS. Anticipatory activity persisted at least for four cycles after interrupting palatable meal, suggesting that this persistence depends on an endogenous oscillator. Glucose and free fatty acids were not entrained in PME rats. c-Fos expression in limbic system nuclei was in phase with PME time, but not in the hypothalamus. Results suggest 1) that food deprivation, i.e. a catabolic state is not necessary for the expression of anticipatory activity; 2) that an increase in the motivational state due to taste and/or nutritional contents of palatable meal is sufficient to entrain behavior; and 3) that structures in the limbic system are involved in this entrainment process. The present study indicates that metabolic and motivational mechanisms are involved in food entrainment, and suggests that the FEO may be a multi-oscillatory system distributed over different regulatory systems in the brain.     

Variety in the diet enhances intake in a meal and contributes to the development of obesity in the rat

Male and female rats were given three palatable, high energy foods either simultaneously or in succession during three 40 min courses. Both simultaneous and successive variety enhanced energy intake compared to the intake of single palatable foods, which was itself enhanced compared to the intake of chow. Rats deprived of food for 24 hr showed a compensatory increase in chow intake (84%) but only a 20% increase in intake in the single palatable food conditions, and no increase in the variety conditions. Male and female rats showed a similar response to variety and deprivation. The effect of variety on body weight was also examined in rats offered either chow, or chow and one palatable food, or chow and three palatable foods in succession (changed every 12 hr) or simultaneously, for seven weeks. All rats offered the palatable foods were hyperphagic compared to chow-fed controls. Rats given the simultaneous but not the successive variety diet were more hyperphagic than the other palatable food groups and showed significantly greater body weight and fat gains. The availability of a variety of foods is an important factor in the amount eaten in the meal and in the etiology of obesity.     

Inhibition of food intake in the rat following complete absorption of glucose delivered into the stomach, intestine or liver.

1. Solutions of glucose or other carbohydrates were administered during the dark or light period of the circadian cycle to rats which had been only briefly deprived of food. 2. food was restored to the animals at various times after administration of a glucose load by stomach tube. With delays between loading and access to food of up to 3 hr by night and 2 hr by day, subsequent food intake was less than intake after non-nutritive loads. 3. measurement of the glucose content of the gastrointestinal tract at various times after glucose loading showed that this depression of intake was still apparent even when the rat was offered food some time after complete absorption of the stomach load. 4. infusion of a glucose solution into the duodenum or the hepatic protal vein also inhibited subsequent food intake. 5. in all cases, the inhibition of food intake was expressed as a decrease in the size of the first meal after restoring access to food. 6. these results provide the first demonstration that the entry of normal amounts of carbohydrate into the body by the physiological route is followed by depression of food intake which lasts until after absorption is complete.     

Plasma corticosterone and meal expectancy in rats: Effects of low probability cues

The plasma corticosterone levels of rats were examined prior to their morning meal on Days 1, 7 and 20 of a regimen of 1 hr access daily to food mash and water. The relationship between external cues and meal provision was varied by feeding some groups immediately upon room entry each morning, and others with a variable interval 90-min delay. On Day 1, corticosterone levels of the hungry rats were elevated around the time of light onset, and until 90 min following room entry. On Day 7, corticosterone levels increased in response to room entry in the delay-fed rats, but did not increase further in immediately fed rats. These results were interpreted as indicating that low, but nonzero, expectancy of meal availability elevates corticosterone levels. On Day 20, the immediately fed rats showed a dip in corticosterone levels after room entry, responding to cues highly predictive of imminent meal availabitity. Delay-fed rats no longer showed an elevation to room entry. The relationship between corticosterone level and meal expectancy is essentially curvilinear in the hungry rat. The corticosterone levels are high when there is uncertainty about whether food is coming or not, and low when there is a very high or very low probability that food is coming.     

Effects of eating,meal cues and ventromedial hypothalamic lesions on serum corticosterone,glucose and free fatty acid concentrations

Male hooded rats were fed once daily for 14 days with 1-hr access to food mash and water to establish the stimuli of the experimenter's entrance into the animal quarters (Experiment 1) or placement of the rat's cage into a chamber (Experiment 2) as cues for a meal. Rats without brain lesions failed to exhibit changes in serum concentrations of glucose or free fatty acids (FFA) to meal cues while exhibiting declines in serum corticosterone concentration. In response to meal consumption, these rats exhibited robust increases in glucose and decreases in FFA and corticosterone levels. VMH lesions (Experiment 2) accentuated glucose declines and diminished corticosterone declines in response to meal consumption or a meal cue. The results suggest a role of the VMH in pituitary-adrenal inhibition, but also indicate the importance of considering the roles of anticipatory metabolic activity and emotional reaction in the regulation of metabolic substrates and pituitary-adrenal activity.     

Central melanocortin receptor agonist reduces spontaneous and scheduled meal size but does not augment duodenal preload-induced feeding inhibition

Central melanocortin (MC) receptor agonists inhibit food intake and may be downstream mediators of the effects of central leptin, which (1) reduces food intake by selectively decreasing meal size and (2) augments the feeding-inhibitory effects of gastrointestinal food stimuli. Central administration of the MC-3/4 receptor (MC-3/4R) agonist, MTII, inhibits feeding in rats, but its effects on meal pattern and potential interactions with gastrointestinal controls of food intake remain unclear. We examined meal patterns and intake in male Sprague-Dawley rats following central intracerebroventricular administration of MTII (0.01-1.0 nmol) in two situations: (1) during daytime 60-min scheduled access to liquid glucose (12.5%) in combination with a duodenal preload of 12.5% glucose or physiological saline (4.4 ml/10 min), and (2) during subsequent overnight access to 45 mg of solid chow pellets. Both duodenal glucose preloads and MTII reduced subsequent glucose intake. However, no dose of MTII augmented the reductions in food intake produced by duodenal glucose alone. During overnight access to pelleted chow, the 0.1- and 1.0-nmol doses of MTII reduced food intake, meal size, meal duration, and body weight, and increased the satiety ratio (duration of intermeal interval/preceding meal size) but did not change meal frequency. The present data (1) demonstrate that MTII, like leptin, reduces food intake by a selective reduction in meal size and not meal frequency, and (2) suggest that MTII increases the feeding-inhibitory potency of negative feedback signals critical to the control of meal size during spontaneous chow access, but not scheduled access to palatable liquid nutrient solutions.     

Interactions between nocturnal feeding and wheel running patterns in the rat

The purpose of this experiment was to investigate the relationship between daily patterns of wheel running activity and feeding behavior. Wheel running and bar pressing for food were continuously recorded from rats which had concurrent access to both wheels and food. In a second condition, wheel running and (non-reinforced) bar pressing were recorded during food deprivation. In a third condition, bar pressing for food was recorded while wheel running was prevented. During ad lib access to wheels and food, both behaviors occurred primarily during the dark phase of the light-dark cycle. However, the nocturnal distribution of these responses differed: the early dark hours were primarily devoted to running, while feeding occurred in small, frequent meals. Maximal food intake occurred during the late dark hours, when meals were larger and less frequent, and there was little running. During three days of complete food deprivation, both wheel running and meal initiation attempts (as indicated by nonreinforced bar pressing) maintained their normal daily patterns. However, during wheel deprivation, changes occurred in the temporal distribution of meals which equalized early and late dark food intake. These results show that the availability of an alternative activity, like wheel running, can be an important determinant of daily feeding patterns. However, the daily pattern of wheel running is not influenced by food availability. Different behaviors may show differential plasticity in the coordination of an overall daily behavioral program.     

The ventromedial hypothalamic nucleus is not essential for the prefeeding corticosterone peak in rats under restricted daily feeding

The role of the ventromedial hypothalamic nucleus (VMH) in the generation of the prefeeding corticosterone peak was examined in rats under restricted daily feeding, under which daily meal supply was restricted to a fixed time in the early light period. Rats were lesioned in the VMH bilaterally and subjected to restricted daily feeding during two different post-operative phases and with two different durations of food presentation. A restricted feeding with free access to meal for 4 hr was imposed on the VMH-lesioned rats from 2 to 4 weeks after the lesion, when the daily food intake increased significantly (dynamic phase). The restricted feeding induced the prefeeding hormone peak in sham operated rats, but failed to develop it in the VMH-lesioned rats. On the other hand, the hormone peak appeared in the VMH-lesioned rats subjected to the feeding schedule from 8 to 10 weeks after the lesion, when the daily food intake was not different from the control (static phase). Moreover, the VMH-lesioned rats showed the hormone peak even in the dynamic phase when the access to meal was shortened to 1 hr. These results indicate that the VMH is not essential for the generation of the prefeeding corticosterone peak under restricted daily feeding, and suggest that a special metabolic state observed during the dynamic phase of VMH lesion prevents the development of the feeding-associated oscillation or its expression upon plasma corticosterone level.     

Phase shifts of circadian rhythms in activity entrained to food access

Rats anticipate daily 2 hr meals with a sharp increase in activity several hours prior to food availability. The present experiment examined the response to phase shifts of food access in rats with lesions of the suprachiasmatic nuclei (SCN). Following entrainment of activity to 2 hr of food per day, food access was phase delayed or phase advanced by 4, 6, or 8 hr. All rats responded to phase delays of 4 or 6 hr with an increase in the duration of anticipatory activity so that transients appeared mostly in activity onset. Following 8 hr phase delays, clear delaying transients in both activity onset and end were observed. Only a few rats showed advancing transients in activity after phase advances of food access. In response to 6 hr and 8 hr phase advances, 3 different responses occurred: (a) activity re-entrained to food access by the 2nd or 3rd day without clear intervening transients, (b) activity phase shifted by means of distinct delaying transients and (c) delaying transients occurred in one component of activity while a second component of activity appeared at the new phase position by the second or third day. These results provide further evidence that anticipation of food access is mediated by a circadian mechanism which is functionally independent of the SCN and illustrate some similarities as well as considerable differences between circadian rhythms entrained by feeding and those entrained by light-dark cycles.     

The role of drinking in the suppression of food intake by recent activity.

The standard activity-based anorexia procedure provides rats with access to a running wheel while restricting their access to dry food. This can produce reduced food intake and progressive weight loss. Using this procedure, in the present study (Experiment 1) the authors found changes in drinking patterns both in the period of high activity preceding food access and during the feeding period. Varying the procedure by providing wet mash (Experiment 2) or by prior adaptation to a drinking schedule (Experiment 3) prevented the self-starvation effect. These results indicate the importance of drinking when analyzing the effect of recent activity on food intake.     

Effects of estradiol on food intake and meal patterns for diets that differ in flavor and fat content

Apart from the well known inhibitory effects of estradiol on food intake, meal size, and body weight in female rats that have been documented over the past thirty years, a more recent report presents the opposite finding; that a large dose of estradiol can increase food intake and weight gain in gonadally intact female rats presented with a palatable diet. The purpose of the present experiment was to further examine this hypothesis by evaluating the ability of estradiol to influence feeding behavior in ovariectomized rats presented with diets that differ in flavor and fat content. Female rats were given a cyclic regimen of estradiol benzoate treatment (5.0 or 20.0 µg) or the oil vehicle and were presented with the standard chow diet or a diet with a higher fat content and chocolate flavor. Food intake, meal size, and meal number were monitored three days after the first injection of estradiol or oil. Compared to the chow diet, food intake increased when animals had access to the chocolate/fat diet during the vehicle treatment condition. Both doses of estradiol significantly decreased food intake, meal size, and body weight gain when animals were presented with either the standard chow diet or the chocolate/fat diet. These findings indicate that estradiol does not stimulate the intake of a palatable diet in ovariectomized rats, and suggest that previous results showing that estradiol enhanced eating and weight gain stemmed from a disruption of the hypothalamic-pituitary-gonadal axis when intact females received a large dose of exogenous estradiol.     

The multiplicity of biological oscillators in the control of circadian running activity in the rat

The purpose of this experiment was to determine the degree to which a circadian running activity could be controlled by multiple biological oscillators within a single organism. Twelve male rats, housed in running wheels, had access to food from noon to 1 p.m. daily. In addition to this noon or fixed feeding, the rats received a moving feeding during which they had access to food for 1 hr every 25 hr. These two circadian feedings had a continually shifting phase relationship to each other. The animals responded to this regime by displaying separate bursts of running activity which were in relative coordination. Each animal's behavior shifted between the three following general patterns: (1) one cycle of activity, either before the fixed or the moving feed; (2) two bursts of activity, one before the fixed feeding and one preceding the moving feeding or (3) a burst of activity not entrained to either one of the feedings but rather free running between the two. When the animals were subsequently offered continuous access to food, 10 of the 12 rats maintained two bursts of activity.     

Persistence of rat nocturnal feeding and drinking during diurnal presentation of palatable diet

Male laboratory rats were fed a highly palatable diet during the light phase of a 12:12 LD cycle. Nocturnality of feeding was reduced from approximately 77% of the total daily intake to 56%. Increase in diurnal feeding was attributable to the ingestion of a large meal at the beginning of the L phase. Reduction of nocturnal feeding was attributable to decreased ingestion in the first 3 hr of the D phase. Outside these times the daily feeding pattern was unmodified. These findings do not represent a reversal of the feeding rhythm as previously reported, but a modification which is not consistent with interpretation as a phase-shift. Throughout the testing period total daily food intake remained unchanged and body weight was defended despite the increase in calories ingested. Daily drinking patterns and total daily water intake were not significantly altered by the experimental manipulation.     

Elevated blood glucose levels and satiety in the rat.

Thirteen rats were placed on a feeding schedule of two 3 hr periods of food availability daily. blood glucose levels of the experimental animals were altered by intraperitoneal injection of 8 ml of 2, 5, 8, 12, 16, 20 and 25 percent glucose solutions and by intragastric loading of 8 ml of 25, 50 and 65 percent glucose solutions in 10 different feeding experiments. In 4 additional experiments, experimental animals received intraperitoneal injections of 8 ml of 12, 16, 20 and 25 percent solutions of mannitol, a non-metabolizable sugar-alcohol. Controls always received identical quantities of mammalian Ringer's solution administered via the same route as in the experimentals. Another set of 20 rats was used to determine glucose tolerance curves for each concentration of glucose and mannitol administered. No food intake depression occurred following intraperitoneal injections of 2, 5, 8 and 12 percent glucose, 12 percent mannitol and following intragastric loading of 25, 50 and 65 percent glucose. However, intraperitoneal injections of 16, 20 and 25 percent glucose and mannitol caused depression of food intake. No depression of food intake occurred following intragastric loading of 50 and 65 percent glucose solutions which raised blood glucose levels a minimum of 43 mg percent and 55 mg percent above basal, respectively, for the duration of the 3 hr feeding period. These minimum blood glucose levels greatly exceed normal values following a meal. It was concluded that blood glucose level per se is not an important feedback parameter in the long-term control of food intake, and the depression in food intake following intraperitoneal injections of 16, 20 and 25 percent glucose and mannitol solutions was due to an abnormal physiological condition.     

Effects of vagotomy on entrainment of activity rhythms to food access

Subdiaphragmatic vagotomy had little effect on anticipatory wheel running in rats maintained on 4 hr of food access per day. In ad lib conditions, sections of the anterior trunk of the vagus reduced total activity, but did not substantially affect its nocturnal distribution. During restricted feeding, rats with anterior trunk sections showed amounts of anticipatory wheel running comparable to controls, while vagotomized rats with suprachiasmatic nucleus (SCN) lesions showed a reduced amount of activity. After the posterior trunk was sectioned, as well, or after combined sections, there was little change in the amount of anticipatory activity but a significant increase (1 hr) in the duration. In control rats, the duration of anticipatory activity showed a nonsignificant decrease over 50 days on restricted feeding. Neither the vagus nerve, nor the SCN, are essential for the entrainment of activity to restricted feeding.     

Spontaneous meal patterns in female rats with and without access to running wheels

Rats display strong behavioral rhythms during the ovarian cycle. During estrus, food intake is minimal due to a decrease in meal size, and locomotor activity is maximal. To investigate how activity influences feeding patterns across the ovarian cycle, we used a computerized system to monitor spontaneous meal patterns in intact, cycling female rats with and without access to running wheels. We found that running wheel access decreased dark meal frequency, increased dark meal size, and increased 24-h water intake during each phase of the ovarian cycle. In contrast, body weight, 24-h food intake, and the ovarian rhythms of reduced food intake, meal size, and body weight during estrus were not affected by running wheel access. In particular, the reduction in food intake during estrus was due to a selective reduction in dark meal size, not dark meal frequency, and this occurred independent of wheel access. These data indicate that estrus-related changes in spontaneous meal patterns and locomotor activity are independently controlled and that the reduction in food intake during estrus involves a selective change in the neurobiological controls of meal size.     

Entrainment of anticipatory activity to various durations of food access

Rats with lesions of the suprachiasmatic nuclei (SCN) and intact rats were maintained on restricted feeding with the duration of food access ranging from 4 to 12 hr. All rats with SCN lesions displayed at least some anticipatory activity (AA) at all food access durations. The amount of AA diminished when food access was extended to 12 hr and was lowest in a group that was exposed only to a 10-hr access period. The onset of AA (phase angle of entrainment) appeared to be more sensitive to the time of food availability than to its termination. Most intact rats maintained in constant light displayed some AA at food access durations between 4 and 10 hr. In most cases the period of the free running rhythm increased as it crossed food access and the free running rhythm became increasingly disrupted as the experiment progressed in all rats. In some cases the free running rhythm appeared to force AA out of entrainment. These results demonstrate that AA occurs in conditions that impose only minor deficits on the energy balance of rats. Furthermore, they provide additional evidence of interactions between two separate circadian pacemaking systems.