Sham-feeding of corn oil by rats: sensory and postingestive factors

Reduced sensitivity to cholecystokinin in male rats fed a high-fat diet is reversible

Adult rats chronically fed a high-fat (HF) diet maintain reduced sensitivity to cholecystokinin (CCK). We hypothesized that, similar to adult rats, pups fed a HF diet would also exhibit reduced sensitivity to CCK. To test this, male pups fed low-fat (LF) and HF isoenergetic (16.2 kJ/g) diets were administered CCK intraperitoneally (0.125-1 microg/kg) 1 wk following dietary adaptation. After receiving 0.5 microg/kg CCK, pups fed the HF diet suppressed food intake less (8.9 +/- 5.0%) than pups fed the LF diet (28.9 +/- 4.7%; P < 0.05) relative to intakes after saline administration. We then assessed the development and extinction of changes in CCK sensitivity by switching the diets between the groups. The HF-fed group, when switched to the LF diet, regained sensitivity by wk 4 and suppressed food intake following administration of 0.25 microg/kg CCK (33.1 +/- 5.7%; P < 0.05). The LF-fed group, when switched to the HF diet, lost sensitivity by wk 2 and did not suppress food intake after administrations of CCK compared with saline. Finally, we examined if HF-fed rats have an increased sensitivity to corn oil during brief access tests using a multibottle gustometer. At oil concentrations of 25, 75, and 100%, rats fed the HF diet sampled more oil than LF-fed rats (P < 0.05). These findings demonstrate that male rat pups fed a HF diet exhibit reduced sensitivity to CCK, the development of this reduced sensitivity is quicker than its extinction, and rats consuming a HF diet have increased oral sensitivity to oils.

Altered orosensory sensitivity to oils in CCK-1 receptor deficient rats

CCK-1 receptor deficient Otsuka Long Evans Tokushima Fatty (OLETF) rats are hyperphagic, which leads to subsequent obesity and diabetes. Additionally, they have increased sham intake and enhanced preference for sucrose solutions relative to control, Long Evans Tokushima Otsuka (LETO) rats. To determine the effects of oil on ingestion, we first measured real feeding of various concentrations of oil emulsions (12.5, 25, 50, 75, and 100%) in rats that were fed ad libitum. Secondly, to isolate the orosensory compontent of oils from post-ingestive consequences, as well as determine the contribution of energy status, we measured sham feeding in OLETF and LETO rats using one-bottle acceptance tests while non-deprived and overnight food deprived. Finally, to assess the orosensory effects of nutritive and non-nutritive oils, we used two-bottle preference tests in sham fed OLETF and LETO rats. We found that real feeding resulted in increased intake of high oil concentrations for OLETF rats relative to LETO rats. Similarly, OLETF rats consumed significantly more of higher concentration corn oils than LETO while non-deprived sham feeding. Conversely, OLETF rats overconsumed low concentration corn oil compared to LETO during overnight deprived sham-feeding tests. In two-bottle sham-feeding preference tests, both non-deprived OLETF and LETO rats preferred corn to mineral oil. Collectively, these results show that increased oil intake in OLETF rats is driven by both peripheral deficits to satiation and altered orosensory sensitivity.

Meal termination using oral sensory satisfaction: a study in non-obese women

Non-obese individuals limit energy intake and maintain weight long-term. Knowledge of the mechanism they use to accomplish this could help in weight maintenance after weight loss and prevention of unneeded weight gain. The objective of the present study was to determine whether non-obese women use orosensory cues to limit short-term intake of high-energy density foods. Forty non-obese women consumed two high-energy rice puddings of equal energy density, one with high sugar and one with high fat. There were four study conditions. Participants ate until satisfied (basal), satisfied based on taste (taste), satisfied based on fullness (fullness) and while watching television (distraction). The participants were asked to refrain from eating and drinking after 2200. The treatments were done between 0830 and 0900. The order of treatments was randomized and all subjects completed all treatments. The quantity consumed and variation in hunger and taste intensity were the primary measures. Results were subjected to within subjects analysis of variance (ANOVA). Intake was significantly greater in the high-fat groups than in the high-carbohydrate groups (P = 0.0001). Study condition had a significant effect on intake (P = 0.0000) with the taste condition generally resulting in the lowest intake. Hunger intensity significantly decreased after meal intake (P = 0.0156). There was a significant effect of condition (P = 0.0001) with base and taste conditions showing the greatest decrease. In the taste condition, specifically, the intensity of taste and hunger declined after intake (P = 0.0000). These results indicate that taste satisfaction can limit meal intake in non-obese women. Taste satisfaction could be a mechanism that is used to reduce food intake to compensate for previous excess consumption.

Effect of high-fat diet during gestation, lactation, or postweaning on physiological and behavioral indexes in borderline hypertensive rats

Maternal obesity is becoming more prevalent. We used borderline hypertensive rats (BHR) to investigate whether a high-fat diet at different stages of development has adverse programming consequences on metabolic parameters and blood pressure. Wistar dams were fed a high- or low-fat diet for 6 wk before mating with spontaneously hypertensive males and during the ensuing pregnancy. At birth, litters were fostered to a dam from the same diet group as during gestation or to the alternate diet condition. Female offspring were weaned on either control or "junk food" diets until about 6 mo of age. Rats fed the high-fat junk food diet were hyperphagic relative to their chow-fed controls. The junk food-fed rats were significantly heavier and had greater fat pad mass than those rats maintained on chow alone. Importantly, those rats suckled by high-fat dams had heavier fat pads than those suckled by control diet dams. Fasting serum leptin and insulin levels differed as a function of the gestational, lactational, and postweaning diet histories. Rats gestated in, or suckled by high-fat dams, or maintained on the junk food diet were hyperleptinemic compared with their respective controls. Indirect blood pressure did not differ as a function of postweaning diet, but rats gestated in the high-fat dams had lower mean arterial blood pressures than those gestated in the control diet dams. The postweaning dietary history affected food-motivated behavior; junk food-fed rats earned less food pellets on fixed (FR) and progressive (PR) ratio cost schedules than chow-fed controls. In conclusion, the effects of maternal high-fat diet during gestation or lactation were mostly small and transient. The postweaning effects of junk food diet were evident on the majority of the parameters measured, including body weight, fat pad mass, serum leptin and insulin levels, and operant performance.

Mercaptoacetate inhibition of fatty acid β-oxidation attenuates the oral acceptance of fat in BALB/c mice

We investigated the effect of β-oxidation inhibition on the fat ingestive behavior of BALB/c mice. Intraperitoneal administration to mice of mercaptoacetate, an inhibitor of fatty acid oxidation, significantly suppressed intake of corn oil but not intake of sucrose solution or laboratory chow. To further examine the effect of mercaptoacetate on the acceptability of corn oil in the oral cavity, we examined short-term licking behavior. Mercaptoacetate significantly and specifically decreased the number of licks of corn oil within a 60-s period but did not affect those of a sucrose solution, a monosodium glutamate solution, or mineral oil. In contrast, the administration of 2-deoxyglucose, an inhibitor of glucose metabolism, did not affect the intake or short-term licking counts of any of the tasted solutions. These findings suggest that fat metabolism is involved in the mechanism underlying the oral acceptance of fat as an energy source.

Influence of oral and gastric NaCl preloads on NaCl intake and gastric emptying of sodium-deficient rats

Evidence is mixed as to whether oral metering contributes to the satiation of NaCl intake. To examine this in detail, we measured NaCl intake of sodium-deficient rats given preloads of NaCl that were sham ingested, normally ingested, or intubated into the stomach. Intake of 500 mM NaCl was reduced by prior ingestion, but not by sham ingestion, of an NaCl preload. NaCl intubation reduced NaCl intake if the test began 15 min, but not 60 min, after the preload. Gastric emptying of NaCl was initially more rapid after intubated than after ingested NaCl. Plasma aldosterone concentrations dropped more rapidly after ingested than after intubated NaCl and also dropped after sham ingestion of NaCl, raising the possibility of a cephalic-phase influence on aldosterone levels. These findings suggest that oral factors do not directly control the amount of NaCl consumed by sodium-deprived rats. Differences between the physiological effects of voluntary ingestion and intubation may be responsible for the results of several early studies purported as evidence for oral metering of sodium consumption.

Opioidergic contribution to conditioned place preference induced by corn oil in mice

We previously reported that voluntary intake of corn oil in the light box showed place preference in the conditioned place preference (CPP) test in mice. In the present study, we investigated the contribution of opioidergic systems to the corn oil-induced CPP in mice. Acquisition of the place preference by corn oil intake was blocked by i.p. injections of an opioid mu antagonist, naloxone (0.1 and 0.3 mg/kg), and delta antagonists, 7-benzylidenenaltrexone (0.5 mg/kg) and naltriben (0.5 mg/kg) 15 min before conditioning. The opioid kappa agonist U-50488H (1 and 3 mg/kg i.p.) also blocked corn oil-induced CPP. Naloxone (1 mg/kg, i.p.) and naltriben (0.5 mg/kg, i.p.) did not affect corn oil intake in the home cage. However, 7-benzylidenenaltrexone (0.5 mg/kg, i.p.) and U-50488H (1 mg/kg i.p.) decreased and increased the corn oil intake, respectively. These results suggested that the rewarding effects of corn oil in the CPP test are at least partially mediated via opioidergic systems through mu and delta receptors. Further, we showed that an opioid kappa agonist reduced the rewarding effects of corn oil in the CPP test in mice, although it increased corn oil intake.

Grooming behavior in mice induced by stimuli of corn oil in oral cavity

Mice show a strong preference for corn oil, which was thought to be elicited by stimulation in the oral cavity. Grooming behavior is known to be induced by dopamine D(1) stimulation in rodents. Therefore, we evaluated stimulation by corn oil in the oral cavity and the contribution of D(1) receptors to corn-oil-induced grooming in mice. Intraoral injection (0.1 ml) of corn oil induced grooming behavior similarly to SKF 38393 (10 mg/kg i.p.), a D(1) agonist, and both were antagonized by pretreatment with SCH 23390, a D(1) antagonist. However, a higher dose was needed for antagonism of the corn-oil-induced grooming compared with that induced by SKF 38393. Long-chain fatty acids, their methyl esters and alcohol, their triglycerides, mineral oil and silicone oil but not glycerin, a short-chain triglyceride, xanthan gum solution, or sucrose solution also induced grooming in mice. Xanthan gum solution, which was suggested to mask oil-like texture, attenuated the silicone-oil- but not corn-oil-induced grooming when injected intraorally as a mixture with an equal volume of the oil (50% suspension). The silicone-oil-induced grooming was reduced by SCH 23390 similarly to that induced by corn oil. These results suggested that stimulation by the oil-like texture in the oral cavity in mice induced grooming behavior and that it might be mediated at least partially via D(1) receptors. Moreover, stimuli other than texture might also contribute to the corn-oil-induced grooming.

Effects of oil intake in the conditioned place preference test in mice

We investigated the effects of corn oil intake in the conditioned place preference (CPP) test in mice. Voluntary intake of corn oil in the light box in the CPP test showed place preference, but its peroral administration 60 min before conditioning did not show either place preference or aversion. Acquisition of the place preference by corn oil intake was blocked by i.p. injection of SCH 23390 (0.03 mg/kg) and haloperidol (0.1 mg/kg), but not by that of (+/-)-sulpiride (100 mg/kg), (-)-sulpiride (100 mg/kg), and L-741, 626 (1 mg/kg) 15 min before conditioning. These results suggest that stimulation of corn oil in the oral cavity, but not its postingestive effects, have positive reinforcing effects and the stimulation of corn oil is at least partly mediated via dopaminergic systems through the D(1) receptors. Moreover, the present results suggest that the CPP test is useful for the study of preferable stimulation and rewarding effects of food intake.

Orosensory factors in the ingestion of corn oil/sucrose mixtures by the rat

The experiments reported here were designed to study the orosensory factors contributing to the ingestion of sucrose/corn oil mixtures. When a flavor aversion was conditioned to the sucrose/corn oil mixture, the subsequent aversion to the mixture strongly generalized to the corn oil but very little to the sucrose. Rats conditioned with corn oil show a more profound aversion to the sucrose/corn oil mixture than rats conditioned with sucrose, indicating that the salient feature of the sucrose/corn oil mixture is the oil. Aversion to the sucrose/corn oil mixture does not generalize to a sucrose/mineral oil mixture, giving evidence that the textural aspects of the oil do not play a major role in its perception. This flavor aversion to the mixture is further illustrated in very short-term tests where postingestive factors are minimized, indicating a role for the gustatory system in the detection of the sucrose/corn oil mixture. Preliminary experiments are reported where conditioning tests were run with mixtures of sucrose and linoleic acid, one of the fatty acids that is possibly derived from a breakdown of the corn oil in the oral cavity by lingual lipase from von Ebner's Gland.

The composition of the maintenance diet alters flavor-preference conditioning by intragastric fat infusions in rats

Prior studies indicate that intragastric (IG) fat infusions condition only weak flavor preferences in chow-fed rats using brief daily training sessions. The present study attempted to facilitate fat conditioning by feeding rats a high-fat maintenance diet or by adding an oily flavor to the conditioning stimuli. In Experiment 1, rats were fed restricted rations of either a chow-corn oil mixture (48% energy as fat, HF group) or regular chow (12% fat, LF group). During 1-bottle training sessions, drinking a flavored (CS+, e.g., cherry) saccharin solution was paired with IG fat (7.1% corn oil emulsion). On other days, an alternate flavor (CS-, e.g., grape) was paired with IG water. In subsequent 2-bottle tests between the CS+ and CS- flavors, the HF rats displayed a stronger CS+ preference than the LF rats (90% vs. 62%). Experiment 2 tested the effect of a semisynthetic HF maintenance diet (48% fat energy), using a conditioning procedure similar to that of Experiment 1. The rats displayed only a moderate (72%) CS+ preference. When switched to the chow-oil maintenance diet and retrained with new CS flavors, they developed a 90% CS+ preference. In Experiment 3, chow-fed rats were trained and tested with oily CSs (i.e., 2% corn oil was added to flavored saccharin solutions). They failed to show a preference for the CS+ paired with IG oil. Thus, increasing the level of fat in the maintenance diet can greatly enhance preference conditioning with IG fat, but the amplitude of the effect is influenced by the composition of the high-fat food. In contrast, adding a fatty flavor to the conditioning stimuli did not improve fat conditioning.

Interaction between orosensory and postingestional stimulation in the control of corn oil intake by rats

The microstructure of the ingestive behavior of rats ingesting corn oil emulsions was analyzed to describe quantitatively how postingestional stimulation interacts with orosensory stimulation to control the intake of corn oil. Seventeen hour food deprived rats were given 30-min access intake tests with corn oil emulsions ranging in concentration from 0.125% to 64%. The volume ingested showed an inverted U shaped function over this concentration range. The Weibull function fitted to the rate of licking function showed that the initial rate of licking was an increasing linear function of the log of the concentration of corn oil. The slope of the licking rate function was an increasing curvilinear function of the log of corn oil concentration. Together these results indicate that the negative feedback effect of corn oil in the gastrointestinal tract increases more rapidly at the higher concentrations of corn oil than does its orosensory stimulating effect resulting in a decrease in the volume ingested. Unlike carbohydrates, where the increase in the initial rate of licking is a result of an increase in the size of the clusters, the increase with concentration in the initial rate of ingesting corn oil was caused by an increase in the number of clusters rather than their size. This study provides a quantitative analysis of the interaction between orosensory and postingestional stimulation, two variables that play an important role in the control of ingestion of corn oil.

No effect of extended home use on linking for sensory characteristics of reduced-fat foods

In order to assess whether and to what extent extended sensory exposure would modify liking for the sensory characteristics of specific reduced-fat food items, consumers were provided with unlabelled full- or reduced-fat (FF or RF) versions of two foods for home use over 12 weeks. Group 1 (n = 30) received unlabelled RF cheddar cheese and FF potato crisps, and group 2 (n = 26) received the alternative versions of these same products. Blind sensory tests on these plus a control item (savoury crackers) were carried out at weeks 0, 3, 6, 9, and 12. A control group (n = 23) participated only in sensory tests. The two versions of crisps did not substantially differ initially, and showed minimal changes in ratings over time. The cheeses differed more substantially in sensory quality, and there were a number of significant changes in pleasantness ratings and selected attribute ratings over time. However, these were largely similar for all groups and therefore not specifically related to the use of the specific products. Group 1 consumed less cheese than group 2, but consumption did not change over time. These data indicate that extended home use did not have any unique facilitating effect on sensory or hedonic responses to these RF foods and suggest that, in the absence of specific cognitive cues or an overall change in the sensory or nutritional composition of the diet, consumers do not exhibit any spontaneous change in liking for the sensory characteristics of RF versions of foods.

Experience with a macronutrient source influences subsequent macronutrient selection

We examined the influence of experience with a macronutrient on subsequent macronutrient selection. For 4 days, rats ate chow, or chow and a single macronutrient source. They then had simultaneous access to protein, carbohydrate (CHO) and fat sources, according to a standard macronutrient self-selection paradigm. When selecting among the macronutrient sources, rats pre-exposed to CHO ate more CHO, and those pre-exposed to fat ate more fat, relative to the other groups. Rats pre-exposed to protein ate more protein than did those pre-exposed to CHO or fat but not more than those that received no macronutrient pre-exposure. These selection patterns persisted for at least 12 days, when the test was ended because of the low protein intakes and poor growth of the rats pre-exposed to the CHO and fat sources. After 34 days of recovery with only chow to eat, the rats were again allowed to choose among the three macronutrients, and their patterns of selection were essentially unchanged. Similar results were found in a second experiment in which a 5-day interval was interposed between macronutrient pre-exposure and macronutrient selection. These findings show that experience with the macronutrients typically used in self-selection experiments can have a large, long-lasting, and sometimes detrimental effect on subsequent food selection by rats. Prior experience can be a more powerful influence than nutritional wisdom in determining the rat's food choice.

Ingestive/aversive response of rats to sweet stimuli. Influence of glucose, oil, and casein hydrolyzate gastric loads

Facial consummatory responses reflecting ingestive and aversive perceptions were studied and quantified in rats chronically implanted with gastric and oral catheters. A gustatory stimulus of 50 microliters of 0.6 mol.l-1 sucrose was injected into the mouth every 5 min during 90 min. At time zero, one of seven loads was injected into the stomach. These consisted of, 5 ml of water, or 5 ml solution containing 1 g glucose, 3 g glucose, 1 g casein hydrolyzate, 3 g casein hydrolyzate, or of oil 0.6 ml, or 1.4 ml. The typical ingestive facial consummatory responses in response to sweet stimuli were observed prior to all gastric loads, and also after the water load. On the other hand, the consummatory responses to sweet stimuli turned aversive after all three high-calorie gastric loads. The magnitude of this decrease in palatability (negative alliesthesia) was similar after glucose, casein hydrolyzate, and oil. The reversal of the consummatory responses from ingestive to aversive did not reach the threshold of statistical significance after the three low-calorie gastric loads. These results would tend to show that the intestinal signal for alimentary alliesthesia is nonspecific.

Role of dietary fat in calorie intake and weight gain

This paper reviews the literature on the role of dietary fat in calorie intake and body weight gain in humans and laboratory animals. An overview of 40 animal studies which compared growth on high-fat (HF) and high-carbohydrate (HC) solid/powdered diets indicated that the HF diet elicited greater weight gain in 33 out of 40 studies. Enhanced growth on the HF diet was often, but not exclusively, attributable to greater caloric intake. Additional evidence for the growth-enhancing effect of HF diets emerges from "diet option" and "supermarket" feeding studies in rats, and experimental and epidemiological studies in humans. Three principal factors that contribute to the different responses to HF and HC diets are (a) caloric density, (b) sensory properties and palatability, and (c) postabsorptive processing. It is concluded that both calorie intake and metabolic energy expenditure are biased towards weight gain when a HF diet is consumed, and that the high caloric density of high-fat diets plays a primary role in weight gain. Humans may be biologically predisposed to gain weight when a HF diet is consumed.

Sham-feeding sucrose or corn oil stimulates food intake in rats

In separate experiments, rats with open gastric cannulas were sham-fed either 32% sucrose solution or 15% corn oil emulsion. The rats' cannulas were then closed, and food intake was measured for 2 h. Food intake was greater after sham-feeding either fluid than after tests when no fluid was available. These results suggest that the oral stimulation produced by ingestion of sweet or oily fluids can stimulate appetite in the rat.

Orosensory stimulation of feeding by oils in preweanling and adult rats

To determine the orosensory control of ingestion by oils, we investigated the acceptance and preference of corn oil and of mineral oil in a series of experiments with preweanling rat pups, adult male, Sprague-Dawley rats, and adult female lean and obese (fa/fa) Zucker rats. Emulsions of corn oil in water (greater than or equal to 25%), administered by anterior intraoral catheter, elicited more intake than water at 14, but not 7, days of age. By 21 days of age, intake was a sensitive function of emulsified corn oil concentrations (greater than or equal to 6.25%). In experiments with adult rats, the sham feeding technique was used to maximize the relationship between orosensory stimulation and intake. Rats sham fed approximately similar volumes of 100% corn oil and 100% mineral oil. Maximal rate of intake was about 1 ml/min. The large intake of mineral oil is compelling evidence for the potency of the orosensory stimuli of oil to control intake in the absence of postingestive metabolic effects. The orosensory control is very sensitive in the adult rat--0.78% corn oil produced a larger intake than water. The orosensory control of acceptance (1-bottle tests) did not correlate with preference (2-bottle tests): 100% corn oil and 100% mineral oil were equally accepted, but rats showed a very strong preference for 100% corn oil. The peripheral sensory mechanism of this orosensory control is unknown. As a working hypothesis, we suggest that oral tactile mechanisms of the trigeminal afferent nerves provide the necessary sensory pathway.(ABSTRACT TRUNCATED AT 250 WORDS)

Relationship of dietary fat content to food preferences in young rats

Weanling rats were fed either a high-fat (30% of calories) or a low-fat (10% of calories) diet for four weeks, after which fat preference was assessed using a choice paradigm. Fat preference was measured during 2-hour intake tests in which three peanut butter/peanut oil mixtures containing 0.50, 0.61, and 0.71 grams fat/gram were offered to each animal. Rats fed the high-fat (HF) diet preferred the highest-fat mixture and consumed more total fat during intake tests than animals fed the low-fat (LF) diet. Intake of NaCl and sucrose solutions was measured during separate intake tests. LF-fed rats drank more NaCl solution than HF-fed rats. Following these tests a subgroup of the LF-fed animals was fed the HF diet, and a subgroup of the HF-fed group was fed the LF diet for a further four weeks. Upon repetition of the intake tests, rats that had been fed the HF diet during the initial four weeks still preferred the highest-fat mixture.

Corn oil and mineral oil stimulate sham feeding in rats

To determine the orosensory effects of oils on ingestion, we measured the 1-bottle intake of corn oil and of mineral oil during 30 minutes of sham feeding in rats that were food deprived overnight or nondeprived. Rats sham fed both oils. Food-deprived rats ingested significantly more of both oils than nondeprived rats. Rats discriminated corn oil from mineral oil and as little as 0.78% corn oil emulsion from water. When rats sham fed 8 dilutions of corn oil, intake was an inverted-U function of concentration with maximal intakes produced by 12.5%, 25% and 50% corn oil emulsions. Despite similar, sometimes equal, intakes of corn oil and mineral oil in 1-bottle tests, food-deprived and nondeprived rats showed a strong preference for corn oil in 2-bottle, sham-feeding, preference tests. The sensory mechanisms that mediate the oral effects of oil on intake and preference are not known, but the olfactory and trigeminal sensory systems are the most likely candidates. Further work is required to characterize the potency, sensitivity, and discriminability of the orosensory effects of oils, the mechanisms that mediate them, and their role in the control of fat intake.