Effects of two body weight reduction regimens on schedule dependent and schedule induced behavior

Fifteen male hooded rats were divided into three equal groups on the basis of body weights. One group was allowed to eat ad lib and constituted the Control Group. The second group of animals was reduced over 7 days and adjusted to 80 percent of the body weight of the animals in the Control Group for a 23 day period and constituted the Adjusted Group. Animals in the third group were reduced to 80 percent of their own predeprivation body weight over 7 days and were maintained at this constant level for a 23 day period and constituted the Constant Group. The two experimental groups were then allowed to feed ad lib for the remainder of the experiment. Both the Adjusted and the Constant Groups were tested throughout restricted and ad lib eating periods on a FI 1 min food reinforcement generator schedule. Schedule dependent lever pressing, schedule induced licking and drinking, food intake and body weights were recorded for the duration of the experiment. Results demonstrate that schedule induced licking and drinking and schedule dependent lever pressing increased in the Constant Group at reduced body weight. Animals in the Adjusted Group did not change significantly on these measures during the same periods. When animals were returned to ad lib feeding conditions, the Constant Group initially increased body weight at a faster rate, ate significantly more, and exhibited significantly increased schedule induced licking and drinking. These initial differences disappeared later during the same period. The two experimental groups did not attain the expected body weight of the normal Control Group during 78 days of ad lib eating. Apparently, when animals are initially deprived rapidly and maintained at relatively constant body weight reduction, a prolonged body weight deficit occurs even though these same animals eat normal amounts of food.     

Absence of post-fast food compensation in the golden hamster (Mesocricetus auratus).

Ad lib food intakes and body weights were measured for hamsters fed one of 4 different diets. Animals were then placed on an intermittent starvation (IS) schedule in which food was available ad lib on alternate days only. Hamsters of both sexes showed little or no post-fast food compensation, i.e., after 24 hr of food deprivation their daily food intake was no greater than their daily intake during baseline testing. These animals lost a large percentage of their initial body weight and many of them died. Other hamsters restricted daily to half-day feeding periods that nearly coincided with the light (L) or dark (D) phases of the illumination cycle also failed to show food compensation; they generally ate no more during D- or L-periods that followed a half day of food deprivation than during D- or L-periods that succeeded a half day of ad lib feeding. These animals lost substantial portions of their initial body weight and many died. Hamsters refed after a 96-hr fast and an 18% loss in body weight also did not increase their food intake substantially above baseline values. In each of these experiments substantial portions of the body weight lost during starvation were not regained during extended ad lib refeeding regimens. These findings contrast strikingly with the behavior of rats tested concurrently; rats showed a dramatic post-fast hyperphagia, rapid recovery of body weight lost during starvation, and a reversal of the normal nocturnal feeding pattern when refeeding began during L-periods. Hamsters' nocturnal rhythms of eating and drinking were remarkably stable in the face of all the experimental manipulations. However, hamsters, as well as rats, were quite effective in compensating for changes in diet density; a 1:1 dilution of a liquid diet produced a prompt doubling in the volume of diet ingested. Impressive but less complete compensation was recorded when solid diets were diluted with inert substances (kaolin, cellulose). Hoarding and perhaps hibernation rather than compensation may have evolved as adaptations to periods of food scarcity. Noncompensation may be related to hamsters' nonresponsiveness to some signal of energy depletion. The possibility of lipogenesis being a rate-limiting step is considered. The desirability of adequate field data as a prerequisite to laboratory analysis of feeding behavior is emphasized.     

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.     

Schedule dependent and schedule induced behavior at reduced and recovered body weight.

Rats maintained at 80 percent ad lib feeding body weight became hyperdipsic during 60 min FI-1 min food reinforcement sessions in standard rat operant test chambers. Animals were then allowed to return gradually to their ad lib feeding weight and were tested again in the same experimental chambers for 18 daily and 9 weekly sessions. Data were obtained for test sessions of 10, 30 and 60 min. When body weight recovered, schedule dependent lever pressing and eating as well as schedule induced licking and drinking occurred at reduced frequencies. The behavior endured throughout the test session and over periods as long as 80 days. These persistent and robust stereotyped behaviors are evoked by specific environmental stimuli during a resultant increased responsiveness.     

Diet-induced thermogenesis and cumulative food intake curves as a function of familiarity with food and dietary restraint in humans.

Cumulative food intake curves and diet-induced thermogenesis were recorded in women during four-course solid-food lunches, consisting of familiar and unfamiliar food, offered in ad lib or restricted portions, in order to assess a possible relation between eating behaviour on the one hand and a reflection of internal processes and subject characteristics on the other. The subjects were characterized as normal weight restrained, normal weight unrestrained, and overweight restrained. A negative relation was found between degree of restraint and deceleration of the cumulative food intake curve during the ad lib courses of any menu, and between degree of restraint and diet-induced thermogenesis (p less than 0.001). Consequently, a positive relation was found between deceleration of the cumulative food intake curves during the ad lib courses of any menu and diet-induced thermogenesis (p less than 0.001). Diet-induced thermogenesis during the first serving of a meal consisting of unfamiliar food was significantly higher in all groups than during the other two times, when meals consisting of more familiar food were served (+0.98%; 0.71%, p less than 0.05).     

Effects of subdiaphragmatic vagotomy on schedule induced drinking and schedule dependent lever pressing

Fourteen male albino rats reduced in body weight were exposed to a daily 1 hr FI-1 min schedule of food reinforcement. When schedule induced licking and drinking and schedule dependent lever pressing stabilized, seven animals were subjected to bilateral subdiaphragmatic vagus nerve transections and the other seven animals were subjected to sham operations. Changes in licking, drinking, and lever pressing were assessed when animals were reduced to 80% of their initial body weight by partial food deprivation and when body weights recovered after the animals were returned to conditions of ad lib feeding. Results indicate that vagotomy produces a significant decrease in schedule induced licking and drinking when animals are at reduced body weight. When vagotomized rats are allowed to recover body weight due to ad lib feeding, lever presses decrease but licking and drinking are not further reduced. The sham operation had no appreciable effects at reduced body weight. When allowed to recover body weight sham animals exhibited significant decreases in licking, drinking, and lever pressing. In addition, the rate of body weight gain and home cage food and water consumption and water consumed in response to hypertonic saline injections were reduced in vagotomized animals when allowed to feed ad lib. Results are discussed in terms of vagal afferent activity from the viscera to the lateral hypothalamus associated with food deprivation and body weight reduction and the maintenance of adjunctive behavior.     

Schedule-induced drinking and other behavior in the rat, as a function of body weight deficit

Six rats were presented with food according to a fixed time 60-sec schedule at 80%, 90% and 100% of their initial ad lib body weight, and with food freely available in the home cage. Water was available at all times. Amount of water consumed during test sessions, time spent drinking, number of bouts of drinking, and bout duration of drinking varied inversely with body weight. Visiting the food tray also varied inversely with body weight, while grooming varied directly. In rats fed ad lib in their home cages, more water was drunk when pellets were intermittently scheduled than when the same number of pellets was freely available in a session of equal duration, but time spent grooming did not vary significantly. Thus, drinking is schedule-induced in satiated rats, but grooming is not. It is suggested that the primary effect of satiation is to reduce drinking, and that grooming increases secondarily to fill the time vacated by drinking.     

Effects of VMH lesions on schedule induced and schedule dependent behaviors

Ten male hooded rats were exposed to an FI 1 min food reinforcement generator schedule at 80% body weight and schedule dependent lever pressing and schedule induced licking and drinking were recorded. When lever pressing, licking, and drinking stabilized the 10 rats were divided into two groups. One group was composed of 4 animals subjected to sham lesion procedures plus one animal with asymmetrical ventromedial hypothalamic (VMH) lesions. The other group was composed of 5 animals with bilateral symmetrical VMH destruction. Results demonstrate that VMH destruction produces a slight transient decrease only in water intake when on schedule at 80% body weight. When animals are returned to ad lib eating and body weight increased and they are returned to the test chamber, the VMH lesion animals display increased licking and drinking. Although VMH lesion animals ate and drank more than controls, they did not eat more in response to food deprivation and did not drink more in response to water deprivation and the intraperitoneal administration of hypertonic saline. The presence or absence of food or water was the determining factor in the overeating or excessive drinking of the VMH lesion animals. Results are discussed in terms of gastrointestinal influences on the hypothalamic mechanisms involved in the production of schedule induced behaviors.     

Effects of 2-deoxy-D-glucose on schedule dependent and schedule induced behavior and ingestion at different body weights.

The effects of intraperitoneally administered 2-deoxy-D-glucose (2DG) at 0, 50, 150, 300 and 600 mg/kg, were studied in four experiments. Experiments 1 and 2 determined the effects of 2DG on schedule induced licking and drinking and schedule dependent lever pressing in rats maintained at 80% and ad lib free feeding body weights during 3 hr on an FI-1 min food reinforcement schedule. Experiments 3 and 4 evaluated the effects of 2DG on free access home cage food and water consumption under the same body weight conditions. Decreases in food and water consumption were observed under both feeding conditions when animals were reduced to 80% free feeding body weight. At ad lib feeding body weight, 2DG administration had no effects on licking, drinking, and lever pressing during FI-1 min scheduled food reinforcement or on free access home cage feeding and drinking. The effects of 2DG on ingestive behavior are discussed in terms of central and peripheral glucosensitive cells and the consequences of blocking glucose utilization at different body weights under different stimulus conditions.     

Refeeding after differential severity of deprivation

Forty-eight female rats were randomly assigned in equal numbers to an eight-day 0, 20, 23, 23.5, 46-hour food deprivation schedule, or complete starvation, followed by two weeks of ad lib refeeding. Two of the completely starved subjects died. Due to decreased metabolic need of the animals after weight loss, daily food intake was considered relative to body weight. Water consumption relative to body weight increased by the end of deprivation except for animals unadapted to deprivation. The major findings during refeeding were: (a) post-starvation anorexia does not occur after any form of starvation (scheduled or complete) when body weight is considered; (b) instead of being highest on the first day of refeeding, food consumption slowly increases to a maximum and then returns to ad lib control level; (c) the more severe the deprivation, the longer time for food consumption to reach its maximum and return to control levels; (d) the more severe the deprivation, the greater the maximal food consumption.     

Overcompensation of food intake following brief periods of food restriction

Four adult female Sprague-Dawley rats, maintained on an ad lib feeding schedule were deprived for either 0, 0.5, 1, 2, 4, 6, or 8 hours during the dark phase of the day-night cycle. It was found that the latency to initiate the first meal following the deprivation was independent of the previous deprivation interval. The animals were found to overcompensate for the periods without food by eating a large initial meal that increased proportionately in size with the duration of food restriction. Furthermore, the animals continued to overeat throughout the day. This deprivation-induced overeating by the animals resulted in an overcompensation in total food consumption that was 21–56% greater than on control days. The role of food intake as a regulator of body weight is discussed.     

Interaction between light- and feeding-entrainable circadian rhythms in the rat

The interaction between light-entrainable rhythms (LER) and feeding-entrainable rhythms (FER) of wheel running was studied by maintaining 16 rats in constant darkness and on ad lib feeding for 14 days, followed by a restricted feeding schedule (2 hr food/day) for 42 days, and 25 days on ad lib feeding. In 4 rats the LER assumed the same period as food access during restricted feeding. The pre-restriction periods of the LER of these rats differed from that of the feeding schedule by 5 min or less. However, since food access did not determine the phase of the LER, food does not appear to be a true Zeitgeber for this rhythm. No synchronization was observed when the two periods differed by more than 10 min, although changes in period or phase of the LER during restricted feeding were observed in all rats. Four hour phase advances or delays of the feeding schedule did not result in consistent effects on the phase of the LER. Two hour light pulses near the onset or end of the LER resulted in phase delays and phase advances of the LER, but had no effect on the FER. In the context of a multioscillator circadian system, these results suggest that light- and feeding-entrainable circadian oscillators are only weakly coupled.     

Effects of lateral hypothalamic lesions on schedule dependent and schedule induced behavior

Rats were reduced to 80 percent body weight and were exposed to an FI 1 min food reinforcement schedule for 30 min daily until lever presses, licks and water consumption stabilized for at least 10 days. Six animals were subjected to bilateral mid lateral hypothalamic (LH) lesions and 6 animals to bilateral posterior LH lesions. Animals were tested for 50 days following the lesions at 80 percent body weight, were permitted to recover body weight, and were tested for an additional 30 days under ad lib feeding and after body weight recovered. Animals were then subjected to the following four home cage tests: food consumption following food deprivation; drinking following water deprivation; insulin induced eating; and salt arousal of drinking. On the basis of the data collected in these tests and lesion locus and size, the 12 experimental animals were divided into 3 groups--asymmetrical, typical and posterior bilateral LH lesion and were compared to a sham lesion control group of 4 animals. The asymmetrical LH lesion group was similar to the sham lesion group except lever pressing was significantly depressed. Typical bilateral lesion animals displayed the usual LH syndrome with prolonged depression of both schedule dependent lever pressing and schedule induced licking and drinking. Posterior bilateral LH lesions produced the most drastic effects on both schedule dependent and schedule induced behavior. Results indicate that the neural mechanism which is involved in schedule induced polydipsia is destroyed by posterior bilateral LH lesions.     

Circadian patterning of feeding,drinking and activity during diurnal food access in rats

Using an “ecologically” relevant approach, the present study investigated (1) the association between feeding and drinking patterns and their circadian organization and (2) changes in general activity (or life-style), during ad lib conditions (fa:LD) and in a situation where access to food was restricted to the light phase (fa:L). Rats were housed in large outcages with nest boxes. Feeding, drinking, activity, outcage and nest occupation were recorded automatically throughout the day-night cycle. Access to food was restricted by a sliding door situated in front of the food hopper. Under ad lib conditions rats were mainly nocturnal, eating 94% and drinking 95% of their daily intakes at night. The patterns of food and water intake were similar, showing a bimodal distribution over the dark phase. During fa:L rats showed an initial large peak in feeding with lights on, followed by a long pause. Thereafter, feeding activity was variable but remained at a low level. The nocturnal drinking pattern persisted. However, 17.5% of daily water intake was meal-associated, compared with 71% during ad lib. Diurnal activity was associated with feeding and nocturnal activity with drinking. Nocturnal outcage and nest occupation patterns were not shifted to the light phase. The experiment demonstrates that rats on an fa:L schedule reduce food and water intake, and body weight, while still retaining circadian characteristics in the temporal distribution of drinking, activity, outcage and nest occupation. Further, although feeding and drinking may be causally related they need not occur in close temporal association. The rapidity of re-establishment of the normal feeding pattern, on return to free-feeding, and the close association with drinking under normal conditions, suggest the existence of a single or two coupled oscillators controlling feeding and drinking.     

Zinc deficiency and anorexia in rats: normal feeding patterns and stress induced feeding

We report the effects of zinc deficiency on normal feeding behavior in rats and the effects of zinc deficiency on stress-induced eating in rats. Zinc deficient (ZD) rats weighed significantly less than their pair-fed and ad lib controls. Zinc repletion allowed improved growth, but ZD rats never displayed catch-up growth compared to their ad lib controls. ZD rats rapidly developed a depressed food efficiency ratio which normalized with zinc supplementation. Food consumption in ZD rats was approximately one-third that of ad lib controls and water intake was also significantly reduced. Mild tail pinch was able to induce feeding in these normally anorexic ZD rats. We conclude that zinc deficiency represents an interesting model of anorexia which may enhance our understanding of appetite regulation.     

Effects of food restriction during concurrent lactation and pregnancy in the rat

The ability of rat dams to cope with food restriction as well as a concurrent lactation and pregnancy was examined by varying the duration, time of onset and severity of the food restriction schedule. Groups of females impregnated in the postpartum estrus and nursing 8 pups were either allowed to feed ad lib, given 60% of an ad lib ration from days 1-7, 1-14, 8-14 or 15-21 postpartum or 40% of an ad lib ration from days 1-7. Food restriction regimens that extended through the estimated time of implantation for females nursing 8 pups (days 1-14 and 8-14) resulted in a prolongation of pregnancy. Food restriction in the first week postpartum did not result in a prolongation of pregnancy and did not affect the number of females carrying the pregnancy to term. Dams food restricted in the third week postpartum had pregnancies of the same length as ad lib females but a smaller proportion of this group gave birth to a second litter.     

Food access schedule and diet composition alter rhythmicity of serum melatonin and pineal NAT activity.

This study investigated the effect of dietary composition and food access schedule on the rhythmicity of serum melatonin and pineal N-acetyltransferase (NAT) activity. Wistar rats maintained on a 12:12 h light-dark cycle were assigned to two dietary groups: a group fed rat chow and a group fed a choice between a protein-rich and a carbohydrate-rich diet. Each dietary group was further divided based on feeding schedule, with food available between 0800 and 1600 h or ad lib access to food. Regardless of dietary condition, total food and carbohydrate intake of rats having free access to food was higher than under the restricted food access schedule. Protein intake of rats fed the dietary choice was lower with the restricted access than in the free access. In rats fed the dietary choice, melatonin levels and NAT activity were significantly decreased with restricted access compared to free access. Such results were not found in rats offered restricted chow. This study suggests that the rhythms of melatonin secretion and NAT activity can be altered by dietary composition.     

The effects of 2-deoxy-D-glucose given via the jugular or hepatic-portal vein on food intake and plasma glucose levels in pigs

Intracellular glucose metabolism is reduced in the presence of its analogue 2-deoxy-D-glucose (2-DG). In the present experiments, 2-DG has been administered to pigs to see if it affects appetite and to test the hypothesis that pigs will respond to intracellular glucoprivation by increasing food intake. During the experiments, the pigs were fed on either a one meal per day or an ad lib regime. 2-DG was either given as a rapid injection or by slow infusion over 2 hr. The site of 2-DG administration was either via the jugular or the hepaticportal vein. Injections of 2.5, 3.75 or 7.5 g 2-DG into the jugular vein of hungry pigs 15 min before a meal produced no effect on food intake. With food available ad lib, there was a reduction in the rate of responding for food shortly after the injection of 2-DG. Later, after a 4–5 hr latent period, there was a significant increase in food intake. In hungry pigs 7.5 g 2-DG intraportally produced no effect on meal size, whereas similar treatment with food available ad lib produced a total suppression of responding for the 2–5 hr period post-injection. A lower dose (3.75 g) intraportally in ad lib fed pigs produced no effect. Injections of 2-DG via either route produced a marked hyperglycaemia.     

Social facilitation of eating: effects of social instruction on food intake.

To investigate whether social influences cause increases in eating behavior, thirty undergraduate psychology students completed a diet diary for three 5-day periods. Subjects were instructed to either eat alone or eat with other people, actively eating with them for two of these periods. For the third period, subjects were instructed to eat as they normally would (with or without other people present). When instructed to eat with others present, subjects overall consumed more food, water, sodium, and alcohol than when they were instructed to eat alone. In the normal condition, food intake was 60% higher when the subjects ate with others present than when they ate alone. These results suggest that social facilitation has a causal influence on eating which increases food intake.     

The effects of subdiaphragmatic vagotomy on circadian corticosterone rhythmicity in rats with continuous or restricted food access

Plasma corticosteroid circadian periodicity was determined in three groups of individual adult male rats. Categories were: postsubdiaphragmatic vagotomy, sham-operated, and controls. Blood was sampled every 4 hours over a 48-hour period with ad lib feeding and after a 13-day period on daytime (0930–1330) restriction of food and water availability. Plasma corticosterone circadian periodicity was normal under ad lib conditions in control, sham-operated, and vagotomized animals. Under food-restricted conditions, both the sham-operated and vagotomized animals exhibited the 12-hour shift in the circadian peak of plasma corticosterone levels that we have previously described in normal animals under such conditions. There were no differences between groups in the total amount of food and water consumed or the percentage of nocturnal food intake on an ad lib feeding schedule. Both vagotomized and sham-operated animals manifested reductions in food intake under conditions of food restriction, although vagotomized consumed less than sham-operated animals. The present data indicate that an intact vagus nerve is not necessary for the establishment of circadian periodicity of plasma corticosterone levels or the shift in the periodicity of corticosteroid secretion produced by a restricted feeding regimen.