The role of a noxious taste in determining food intake in the rat

Adult female rats were given access to cornstarch, fat, and 2 cups of casein ad lib. Sucrose octaacetate was added to one or both casein cups in concentrations of 0, 0.1, or 2.5%. The lower concentrations of SOA had no effect on total caloric intake or food choices, although a taste aversion experiment indicated that the rats could detect SOA at this level. The 2.5% concentration had no effect on total caloric intake. If only one casein cup was treated with SOA, the animals consumed 70% of their protein from the unadulterated casein supply, but total protein intake was unchanged. If both cups were treated with 2.5% SOA, protein intake decreased, but carbohydrate consumption showed a compensatory increase. Fat intake was unchanged.     

Increase of food intake induced by glucagon in the rat

The effect of intraperitoneal administration of saline, glucose (25 mg/100 g b.w.), insulin (0.025 U/100 g b.w.) and glucagon (50 micrograms/kg b.w.) on glycemia, liver glycogen concentration and food intake was studied on 104 male adult Wistar rats. When saline was injected the amount of food ingested was similar to that expected at the metabolic moment selected for the tests. Glucose administration did not reduce food intake but both insulin and glucagon provoked a threefold increase during the 60 minutes ensuing the injection. The overall ingestion of food during the 24 hours after the injection of the hormones was significantly higher (about 10%) than the control values during the preceding or the succeeding 24 hours. A hyperphagic, rather than a hypophagic effect of glucagon administration is possibly related to the small dose used in the experiments. The mechanisms involved in the increase of food intake due to glucagon are discussed in terms of acceleration of the metabolic reactions that normally prevent large drops of glycemia as glucose utilization proceeds during the inter-meal periods and that in physiological conditions build up until the need for food arises.     

Regulation of calorie intake in rats with rostral zona incerta lesions: effects of caloric density or palatability of the diet

Following an initial period of hypophagia, the body weights of rats with zona incerta lesions approached asymptotic levels that were significantly lower than those of controls although daily food intake was not reliably depressed. In spite of persisting deficits in feeding efficiency, the ad libitum food intake of the ZI-lesioned rats (relative to body weight) recovered to preoperative levels and was regulated accurately with respect to the caloric density of the diet. However, rats with ZI lesions were finicky eaters and did not maintain stable calorie intake when given unpalatable diets. The water intake of ZI-lesioned rats remained consistently below that of neurologically intact rats but was normal relative to their own food intake and lowered body weight.     

A high-protein diet enhances satiety without conditioned taste aversion in the rat

In order to determine the respective roles of conditioned food aversion, satiety and palatability, we studied behavioral responses to a 50% total milk protein diet, compared with those to a normal protein diet containing 14% total milk protein. Different paradigms were employed, including meal pattern analysis, two-choice testing, flavor testing, a behavioral satiety sequence (BSS) and taste reactivity. Our experiments showed that only behavioral and food intake parameters were disturbed during the first day when an animal ate the high-protein (P50) diet, and that most parameters returned to baseline values as soon as the second day of P50. Rats adapted to P50 did not acquire a conditioned taste aversion (CTA) but exhibited satiety, and a normal BSS. The initial reduction in high-protein diet intake appeared to result from the lower palatability of the food combined with the satiety effect of the high-protein diet and the delay required for metabolic adaptation to the higher protein level.     

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.     

Effects of electroconvulsive shock on food and water intake in the rat

Forty-two male rats were habituated to laboratory conditions for four weeks under constant light conditions. Food and water intakes were recorded daily. Following the habituation period half of the animals were given electroconvulsive shock (ECS) and half Sham-ECS (SECS). ECS treatments produced significant decrements in both food and eater intakes which returned to baseline levels after three days.     

Contrasting effects of systemic and central sibutramine administration on the intake of a palatable diet in the rat

Sibutramine hydrochloride monohydrate is the only centrally active weight-modifying agent currently approved by the FDA for long-term use in the treatment of obesity. Systemic sibutramine treatment has been shown to reduce food intake in humans and rodent models in a manner that is consistent with the enhancement of satiety mechanisms. Although it is generally assumed that the hypophagic effects of the drug are mediated by actions within the brain, the locus or loci of these effects remains unclear. These experiments compared the effects of systemic and intracranial injections of sibutramine on the intake of a palatable diet in non-deprived animals. Consistent with prior reports, systemic injections of sibutramine hydrochloride (at 0, 0.5, 1.0, or 3.0 mg/kg sibutramine i.p.) dose-dependently reduced feeding on a high fat/high sucrose diet across a 2-h feeding session, but did not alter water intake or locomotor activity. In contrast, bilateral injections of sibutramine (at 0.0, 2.0, 4.0 and 10.0 μg/0.5 μl/side) into either the paraventricular nucleus of the hypothalamus (PVN) or the medial nucleus accumbens shell (ACb) significantly and dose-dependently increased food intake of the sweetened fat diet. ACb treatment also modestly inhibited locomotor behavior; intracranial injections had no effect on water consumption. These experiments are the first to suggest that sibutramine treatment may have distinct actions upon separate neural circuits that modulate food intake behavior in the rat.     

Recall of recent lunch and its effect on subsequent snack intake

Recall of food eaten at lunch decreases afternoon snack intake, suggesting that awareness of a recent eating episode may be an important factor influencing appetite. The aim of the present studies was to investigate whether the inhibitory effect of remembering a recent lunch meal on subsequent snack intake is dependent upon 1) the type and palatability of the snack offered; 2) participants' level of dietary restraint and tendency towards disinhibition; and 3) the delay between lunch and recall. Popcorn snacks differing in amount of added salt and rated palatability were offered to male participants in Experiment 1. Participants who recalled the lunch they had eaten that day consumed less of all types of popcorn than participants who recalled lunch eaten the previous day, suggesting that the effect of recent meal recall is not dependent upon the palatability of the snack food. In Experiment 2, a similar pattern of results was observed but only for women who scored low, and not high, on a measure of tendency toward dietary disinhibition, possibly because a tendency toward disinhibition is associated with impaired memory for the lunch. In Experiment 3, decreased cookie intake by women was observed after remembering today's lunch relative to a neutral control condition, but this effect was similarly only observed for participants scoring low in tendency toward disinhibition. In addition, the effect was dependent on the time elapsed between the lunch and recall, since intake was only reduced at a snack tasting session 3-hours post-lunch (when some forgetting of the meal occurred) and not 1-hour post-lunch. It is concluded that the inhibitory effect of recalling foods eaten at lunch on subsequent snack intake is a robust phenomenon that is related to memory of that lunch and is moderated by tendency toward dietary disinhibition.     

Energy density, diet composition and palatability: influences on overall food energy intake in humans

This paper considers the role of energy density (ED), diet composition and palatability in the control of energy intake (EI) in humans through several related considerations: (i) the relationship between ED and diet composition, (ii) the relationship between ED, diet composition and EI, (iii) the relationship between palatability and EI, (iv) the relationship between ED, palatability and EI, (v) the importance of postingestive factors in influencing palatability in the longer term, (vi) the contribution of sensory and nutritional factors to dietary hyperphagia and (vii) the implications these considerations have for people living their normal lives in their natural environment. The main factors influencing ED are the fat and water content of foods. Energy density does elevate EI, especially in short-term studies where it can account for >40% of the variance in EI. In real life, ED accounts for only approximately 7% of the variance in EI. This is because the determinants of EI are multifactorial and also because the short-term effects of ED on EI do not translate into the longer term. We argue that part of the longer term amelioration of short-term effects of ED on EI is due to learned compensation, based on the postingestive consequences of consuming familiar food that differ in ED. More energy-dense foods tend to be more palatable but we learn to consume them in smaller portion sizes. In the longer term, the perceived palatability of a food is strongly influenced by the postingestive consequences of eating it. This effect can override sensory factors alone. This implies that nutrient mimetics, if used continuously, would not be as efficacious as initially supposed and that their ad hoc use may undermine the stability of learned appetites and satieties for foods with different EDs and contribute to the poor weight control capability exhibited by consumers at large.     

Some determinants of intake of glucose+saccharin solutions

In the present three experiments, tests were run to see if postingestional factors played a role in the excessive drinking elicited by the combination of 3% glucose and 0.125% saccharin solutions. The first experiment showed a marked increase in the total daily caloric intake and a decrease in the proportion of calories taken from the food when the glucose+saccharin (G+S) solution was presented to rats. In spite of this alteration of diet induced by G+S drinking, Experiment 2, which compared the drinking of various concentrations of glucose solutions and G+S solutions, demonstrated that the rat drinks these large quantities of the G+S solution for taste, not calories. The final experiment compared preference tests on various concentrations of saccharin, glucose or G+S solutions and showed that preference data across concentrations of G+S were qualitatively similar to the saccharin preference data, and different from the glucose data, even though it was the amount of glucose in the concentrations that was manipulated.     

Effects of palatability and learned satiety on energy density influences on breakfast intake in humans

The present report explored firstly how palatability modified the effects of energy density (ED) on short-term food intake and changes in rated appetite within a single test meal, and secondly how repeated consumption altered these relationships. Experiment 1 contrasted disguised high (HED) and low (LED) versions of a food presented in bland and palatable forms. Mass consumed varied as an interaction of palatability and ED, with subjects eating least of the bland/HED version, suggesting some un-learned satiating effects. No such compensation for ED was seen in the palatable/HED condition, and overall energy intake increased with ED. Palatability had the expected stimulatory effect on appetite, but rated hunger decreased more rapidly as a function of energy consumed in the HED conditions. Experiment 2 introduced novel distinctive flavours to examine whether repeated experience of palatable HED and LED versions resulted in learned satiety. Participants ate the same mass of LED and HED versions on first exposure, but after two training days with each food, where they consumed a fixed amount, they subsequently ate a greater mass of the LED version, consistent with learned satiety. Increased intake was accompanied by a slower rate of decline in hunger in the LED condition. Despite these changes, energy intake remained higher with the HED version. Liking for the LED version was greater than the HED version at the end, possibly due to mild aversive qualities of eating a fixed portion of the HED food during training. Together these data suggest that energy density is the major determinant of short-term energy intake in the absence of orosensory cues predictive of energy differences, but that learning of flavour-energy associations can, to some extent, allow short-term energy consumption to be regulated.     

The ecological determinants of reinforcement in the rat

The intake of a freely feeding animal can be characterized by four parameters: frequency of meals, duration of meals, rate or intensity of eating, and choice of items eaten. The values of these parameters characterize species and the ecological niche they occupy. Constraints on any one of these parameters leads to compensatory changes in other parameters over the feeding cycle. Any change in performance of the instrumental behavior ancillary to feeding which causes the values of these parameters to recover their optimum or privileged value, defined by their niche, will be strengthened. These changes in performance reflect what is usually meant by motivation and the process of reinforcement.     

The effects of liver denervation on food and water intake in the rat

In Experiment 1, liver denervations or sham operations were performed on rats in two separate trials. Food and water intakes and body weights of denervated rats did not deviate significantly from their sham operated controls. Male and female rats responded similarly. In Experiment 2, in addition to daily food and water intakes, initial daily meal size was investigated in two groups of liver denervated and sham operated rats. Initial daily meal size was determined during a one hour test at the start of the dark period of the light:dark cycle. In one group of rats a chow diet was used for testing, while a liquid diet was utilized in the second group. According to the “liver glucoreceptor preabsorptive satiety and general food intake control hypothesis” the denervated rats should have experienced a depression of daily food intake and preabsorptive satiety. No differences were found in either preabsorptive satiety or daily food and water intakes and body weights when denervated rats were compared to sham operated control animals. The results question whether liver glucoreceptors have any detectable influence on the control of feeding behavior. Certainly the data does not support the role of liver glucoreceptors as a major controller of feeding behavior in the rat.     

Sexually dimorphic effects of forced exercise on food intake and body weight in the rat

The food intake (FI), body weight (BWt) and water intake (WI) of adult male and female rats were compared during a seven day period of forced exercise in a treadmill. Work loads for the exercised groups were gradually increased across the seven-day test period, whereas work loads for the sedentary controls were maintained at the same level used during a previous three-day training period. Relative to their respective control groups, male rats showed a decrease in FI and BWt in response to the exercise challenge, but female rats showed an increase in FI sufficient to maintain their BWt at control levels. Both male and female rats showed a reliable increase in WI during the period of forced exercise. These sexually dimorphic changes in FI and BWt in response to forced exercise indicate that female rats are capable of demonstrating a more vigorous defense of BWt than are male rats and suggest that there is a sex difference in the long-term control of feeding behavior in the rat.     

Subcutaneous glycerol injection fails to reduce food intake in rats fed a high protein diet

To test whether glycerol utilization might contribute to glycerol-induced hypophagia, cumulative food intake was measured following subcutaneous glycerol injection in rats fed a high carbohydrate (HC), a high fat (HF) or a high protein (HP) diet, because food composition is known to influence the activity of several glycerol metabolizing enzymes. In additional experiments, the effect of glycerol injection on plasma glycerol, blood glucose, and liver glycogen levels was also investigated in HC-, HF-, and HP-rats. Subcutaneous glycerol injection (6.3 mmoles/kg b.w.) significantly reduced food intake in HC- and HF-rats, but failed to decrease feeding in HP-rats. Five hours after glycerol injection, plasma glycerol levels were significantly elevated in HP-rats, but did not differ from the glycerol levels of controls in HC- and HF-rats. Subcutaneous glycerol injection did not affect blood glucose concentration or liver glycogen content 5 hours after injection in any feeding group. The results suggest that oxidation of the injected glycerol might contribute to the glycerol-induced hypophagia in HC- and in HF-rats.     

Sodium and water intake are not affected by GABAC receptor activation in the lateral parabrachial nucleus of sodium-depleted rats

The activation of GABAergic receptors, GABA_A and GABA_B, in the lateral parabrachial nucleus (LPBN) increases water and sodium intake in satiated and fluid-depleted rats. The present study investigated the presence of the GABA_C receptor in the LPBN, its involvement in water and sodium intake, and its effects on cardiovascular parameters during the acute fluid depletion induced by furosemide combined with captopril (Furo/Cap). One group of male Wistar rats (290–300 g) with bilateral stainless steel LPBN cannulas was used to test the effects of a GABA_C receptor agonist and antagonist on the fluid intake and cardiovascular parameters. We investigated the effects of bilateral LPBN injections of trans-4-aminocrotonic acid (TACA) on the intake of water and 0.3 M NaCl induced by acute fluid depletion (subcutaneous injection of Furo/Cap). c-Fos expression increased (P<0.05), suggesting LPBN neuronal activation. The injection of different doses of TACA (0.5, 2.0 and 160 nmol) in the LPBN did not change the sodium or water intake in Furo/Cap-treated rats (P > 0.05). Treatment with the GABA_C receptor antagonist (Z)-3-[(aminoiminomethyl)thio]prop-2-enoic acid sulfate (ZAPA, 10 nmol) or with ZAPA (10 nmol) plus TACA (160 nmol) did not change the sodium or water intake compared with that for vehicle (saline) (P > 0.05). Bilateral injections of the GABA_C agonist in the LPBN of Furo/Cap-treated rats did not affect the mean arterial pressure (MAP) or heart rate (HR). The GABA_C receptor expression in the LPBN was confirmed by the presence of a 50 kDa band. Although LPBN neurons might express GABA_C receptors, their activation produced no change in water and sodium intake or in the cardiovascular parameters in the acute fluid depletion rats. Therefore, the GABA_C receptors in the LPBN might not interfere with fluid and blood pressure regulation.     

The effects of long warm and cold ambient exposures on food intake water intake and body weight in the capsaicin desensitized rat

Seventeen Sprague Dawley rats received, subcutaneously, 250 mg·kg^–1 of capsaicin divided into 10 increasing doses (10–50 mg·kg^–1) and administered on 7 successive days. Nine controls were treated with an isotonic saline solution using the same protocol. The rats spent, in succession, 5 weeks at 20° C, 6 weeks at 33.5° C, 6 weeks at 8° C, 4 weeks at 30° C and, finally, 5 weeks at 20° C ambient temperature. Their mean food intake (FI), water intake (WI) and body weights were recorded daily. In the 2 groups of rats, FI was inversely related to ambient temperature. However, during the first few days of the exposures, FI in treated rats was greater than controls in the warm environment and less in the cold environment. In controls, WI increased linearly with ambient temperature in the warm environment. This relation was not found in treated rats: they drank less water than controls and lost body weight. During the first days at 8° C ambient temperature, rectal temperature decreased in treated rats and two animals died.The results are similar to those described for rats with hypothalamic lesions. They may also be related to a peripheral effect of the drug.This work was supported by the Centre National de la Recherche Scientifique (C.N.R.S. L.A. 181) and by the Institut National de la Santé et de la Recherche Médicale (I.N.S.E.R.M., A.T.P. 80-79-112)     

Effects of ventromedial hypothalamic lesions on rat ethanol intake

The consumption of food, water and ethanol solutions, in free choice condition, was studied in 9 rats with electrolitic lesion of ventromedial hypothalamus, before and after the operation and compared with 2 controls submitted to sham operation. Changes in body weight were studied between 20 and 30 days after the operation. In all the experimental rats there was an increase in food, water consumption and body weight while ethanol intake decreased significantly.     

Peripheral control of food intake: interaction between test diet and postingestive chemoreception

The experiments reported here were designed to investigate the role of the duodenum in conjunction with the palatability of the diet in the control of meal size. Rats fitted with chronic cannula resting in the duodenal cap were infused with 15 ml of 5.05% glucose, 10.1% glucose, 1.6% urea, 3.2% urea, 15.15% glucose and 4.8% urea at a rate of 0.2 ml/min, or were not infused at all. In one experiment, intake of sweetened condensed milk, and in a second experiment, intake of a 3.2% glucose solution was recorded over a 5 $\text{1}\text{2}$ hour period. Only infusions of very concentrated solutions had significant effects on milk ingestion. On the other hand glucose infusions isosmotic to body fluids led to a decrease in the size of the first meal of a 3.2% glucose solution and a lengthening of the subsequent intermeal interval. These effects were directly related to the concentration of the glucose infused. Equiosmolar urea infusions had no consistent effect on glucose drinking. Intake of moderately palatable substances can be altered by the presence of small amounts of glucose in the duodenum, but intake of highly palatable substances remains largely unaffected unless high concentrations of glucose are infused.     

Palatability, food intake and the behavioural satiety sequence in male rats

Food intake is influenced not only by nutritional status but also by diverse environmental factors. Indeed, a unique quality of food reward is its strong modulation by palatability cues, such as taste, with animals generally preferring diets that are sweet and avoiding those that are either bitter or sour. As appetite suppressants (including those currently in development) could alter food intake by modifying taste sensitivity and/or palatability, the aim of the present study was to characterise the influence of taste adulteration on the normal structure of feeding behaviour, i.e., the behavioural satiety sequence (BSS). Adult male rats were initially habituated both to the basic test diet (mash) and the test arena. Following stabilisation of basal intake, a continuous monitoring technique was used to profile behaviour in weekly 1-h sessions during which the animals were presented, in counterbalanced order, with the basic diet (control) or one of four taste-adulterated variants (0.015% quinine, 0.04% quinine, 0.2% saccharin, 0.3% saccharin). Food intake was strongly suppressed by the higher quinine concentration but was not significantly altered by any of the other additives. Behavioural analysis revealed that this anorectic-like response to 0.04% quinine-adulterated food was associated with a significant reduction in the peak feeding response, highly atypical intermittent food sampling/digging and the virtual absence of resting behaviour. Importantly, this pattern of behavioural change is readily distinguishable from those seen in response to other manipulations that reduce intake, including selective anorectics, sedatives and psychostimulants. Despite the lack of significant effect on food intake or the duration of feeding behaviour, dietary adulteration with 0.015% quinine (and, to a lesser degree, 0.3% saccharin) produced some effects on behavioural structure/time course consistent with a mild aversive response, i.e., bouts of midsession food sampling and a delay in the transition from eating to resting. Data are discussed in relation to the specific behavioural signature to quinine-induced anorexia and its potential utility in identifying appetite suppressants that may modify intake via changes in taste sensitivity and/or palatability.