CD36 gene deletion reduces fat preference and intake but not post-oral fat conditioning in mice.

Several findings suggest the existence of a "fatty" taste, and the CD36 fatty acid translocase is a candidate taste receptor. The present study compared fat preference and acceptance in CD36 knockout (KO) and wild-type (WT) mice using nutritive (triglyceride and fatty acid) and nonnutritive (Sefa Soyate oil) emulsions. In two-bottle tests (24 h/day) naive KO mice, unlike WT mice, displayed little or no preference for dilute soybean oil, linoleic acid, or Sefa Soyate emulsions. At high concentrations (2.5-20%), KO mice developed significant soybean oil preferences, although they consumed less oil than WT mice. The postoral actions of fat likely conditioned these preferences. KO mice, like WT mice, learned to prefer a flavored solution paired with intragastric soybean oil infusions. These findings support CD36 mediation of a gustatory component to fat preference but demonstrate that it is not essential for fat-conditioned flavor preferences. The finding that oil-naive KO mice failed to prefer a nonnutritive oil, assumed to provide texture rather than taste cues, requires explanation. Finally, CD36 deletion decreased fat consumption and enhanced the ability of the mice to compensate for the calories provided by their optional fat intake.

Flavor preferences conditioned by intragastric fructose and glucose: differences in reinforcement potency.

Many prior conditioning studies indicate that fructose, unlike glucose, has minimal postingestive reinforcing effects. Using a new training procedure, food-restricted rats were trained in alternate 20-h/day sessions with one flavored solution (CS+F) paired with intragastric (IG) infusions of 16% fructose and another flavor (CS-) paired with IG water. In subsequent two-bottle tests they showed a robust (85%) preference for the CS+F over the CS-. A third flavor (CS+G) was then paired with IG 16% glucose, and it was strongly preferred to the CS+F. When retrained 30 min/day with new flavors paired with IG fructose, glucose, or water the rats learned only a CS+G preference. When training was extended to 20 h/day, a CS+F preference developed. New rats trained 20 h/day with two-bottle access to CS+F and CS- paired with IG fructose and water failed to acquire a CS+F preference. Other rats rapidly developed a strong preference when trained with concurrent access to CS+G and CS- paired with IG glucose and water. These data indicate that both fructose and glucose generate postingestive reinforcing signals, but that the fructose signals are weaker and/or delayed relative to those produced by glucose.

Flavor preferences conditioned by intragastric infusion of ethanol in rats.

Sprague-Dawley rats were trained 22 h/day to associate a flavored solution [conditioned stimulus (CS+)] with intragastric infusions of 6% ethanol and another flavored solution (CS-) with water infusions. The infusions were matched to the CS intakes so that the animals determined their timing and size. In Phase 1, chow and water were available ad libitum, and both CS flavors were initially sweetened with saccharin that was then faded out. The rats displayed a preference for the CS+ over the CS- under both reinforced and extinction conditions. When food-restricted in Phase 2, the rats displayed an increased preference for the CS+. In Phase 3, the rats were fed ad libitum chow and given preference tests with the CS+ paired with ethanol infusions of increasing concentration (6%, 12%, 18%, and 24%). Their preference for the CS+ over the CS- persisted, and self-administered ethanol dose increased with concentration to 5 g/kg/day. The ethanol-based conditioned flavor preference resembled those conditioned by carbohydrate and fat infusions, suggesting that at least some of reinforcing ability of ethanol may be related to its postingestive nutritive effects.

Conditioned flavor preferences: evaluating postingestive reinforcement by nutrients.

The acceptance of food and the expression of preferences for particular foods have both innate and learned components. To determine the mechanism(s) responsible for the acquisition of learned preferences and the enhancement of inborn taste preferences, it is important to separate the component stimuli: the oral flavor cues and the postingestive nutrient cues that are associated with the flavors. Unambiguous analysis of postingestive reinforcement of a nutrient requires post-oral presentation of the nutrient during or after oral intake of the flavor stimulus. This protocol concentrates on controlling stimulus presentation in associative conditioning, pairing a flavor with positive postingestive effects.

Palatability and foraging cost interact to control caloric intake.

The combined effects of meal cost and food flavor on meal size were studied with a method that avoided the covariation of nutrient composition and caloric density with palatability. As rats (Rattus norvegicus) drank flavored fluids (unpalatable 0.05% sucrose octaacetate [SOA], neutral 0.05% saccharin, and palatable 2% Polycose + 0.2% saccharin [P + S]), liquid diet was infused intragastrically. Relative to saccharin, rats with free access ate 10% more calories in larger meals while consuming P + S and initially ate fewer calories in smaller but more frequent meals while drinking SOA. Other rats lever-pressed to begin meals, which halved meal number and doubled meal size relative to the free-access group. Although foraging rats also ate larger P + S meals and smaller SOA meals, the changes did not affect total intake. Without the usual differential postingestive effects of foods that differ in palatability, making food more costly blunts rats' response to its flavor.

High-fat diet preference and overeating mediated by postingestive factors in rats.

The role of postingestive factors in the preference for and overconsumption of high-fat (HF) foods, relative to high-carbohydrate (HC) foods, was investigated using a self-regulated intragastric feeding procedure. On one-bottle training days, rats drank one flavored saccharin solution [conditioned stimulus (CS) + HF] paired with intragastric infusions of an HF liquid diet, a second flavored solution (CS+HC) paired with an HC liquid diet, and a third flavored solution (CS-) paired with intragastric water. The diets had the same energy and protein content; the CS solutions and infusions along with chow were available ad libitum. The rats drank more CS and self-infused more diet on HF than HC training days. In two-bottle choice tests, the rats preferred the CS+HF to the CS+HC and both CS+HF and CS+HC to the CS-. The rats consumed more CS+HF than CS+HC by taking more bouts per day; bout sizes did not reliably differ. In a subsequent experiment, rats preferred the CS+HF even though diet intakes in training were matched. In a final experiment, the CS+HC and CS+HF intakes were equated in training by diluting the HC diet. Now the rats did not reliably prefer the CS+HF to the CS+HC, yet caloric intakes were much higher on CS+HF than CS+HC training days. Thus, relative to an isocaloric HC diet, the postingestive effects of HF diets stimulate overeating and condition a stronger flavor preference. Reduced satiety rather than increased reinforcement may be the direct promoter of overeating. However, postingestive reinforcement may enhance the selection of HF foods when a choice of HF and HC foods is available.

Preference conditioning alters taste responses in the nucleus of the solitary tract of the rat.

Aversive conditioning has an impact on the neural signal for the gustatory conditioned stimulus (CS). Here, we determined whether the code is also affected by preference conditioning. We paired the taste of MgCl2 (CS+) with intragastric nutrients in some rats (MG), and citric acid (CS+) with nutrients in others (CI). A control group (Control) experienced both tastants without nutrients. Preferences (>90%) developed for each CS+. We recorded responses to 16 taste stimuli in the nucleus of the solitary tract. Responsiveness of acid-oriented neurons to MgCl2 in MG rats was lower than in Controls, and its profile was more distinct from those of acidic and bitter stimuli. Total activity to citric acid was unchanged in CI rats. However, its temporal profile showed a decreased phasic component, making citric acid temporally distinct from nonsugars. Therefore, the responses to both CS+ were modified, each in its own manner, to be more distinct from those of aversive stimuli. The effects of preference conditioning, however, were weaker than those of aversive conditioning.

Diabetic rats prefer glucose-paired flavors over fructose-paired flavors.

Prior studies indicate that glucose has a more potent postingestive reinforcing effect than fructose. The role of insulin in this effect was examined by comparing sugar-conditioned flavor preferences in normal and streptozotocin-diabetic rats. In Experiment 1, diabetic rats, like normal rats, preferred a cue flavor that had been mixed into 8% glucose solution over a flavor mixed with 8% fructose. Both taste and postingestive properties of glucose may have contributed to this preference. Experiment 2 evaluated postingestive reinforcement by pairing cue flavors with intragastric infusions of glucose and fructose. Both diabetics and normals acquired a preference for the flavor paired with intragastric 16% glucose infusions over the flavor paired with 16% fructose infusions although the preference was somewhat smaller in the diabetic rats. Taken together, the results indicate that a normal insulin secretory response to glucose is not required for glucose-conditioned flavor preference. The diabetic rats' reduced flavor preference in Experiment 2 suggests that insulin may play some role in glucose conditioning although this may be secondary to alterations in gastrointestinal motility characteristic of diabetic animals.

Rats integrate meal cost and postoral changes in caloric density.

Rats can maintain energy intake in response to changes in the caloric density of food by compensatory shifts in the number and size of meals. This study challenged rats' regulation of intake in two ways: manipulating the caloric density covertly to remove orosensory cues and altering baseline meal patterns by increasing the cost of obtaining meals. Rats with gastric catheters were maintained on a complete liquid diet delivered by pumps to a drinking spout and to the stomach; within meals, a rat's licking at the spout controlled the timing and duration of the simultaneous isovolemic intragastric infusions. Prior to each meal, some of the rats were required to press a bar to activate the spout mechanism; when the meal cost was paid, a cue light signaled that licks would deliver food. The light remained on until the rat ceased licking for 10 consecutive min; to resume feeding, the rat had to pay the cost again. The standard diet pumped to the spout was prepared at 1 kcal/g; the intragastrically co-infused diet varied from 0 (water) to 2 kcal/g in 0.5-kcal/g increments. Each concentration was presented for at least 4 days. Separate groups of rats were studied at no cost, low cost (5 bar presses/meal), and high cost (80 bar presses/meal). As the infusate concentration increased, the rats ate fewer meals per day. Meal size by weight decreased as caloric density increased, with he largest meals taken when water was infused. Caloric meal size increased linearly with caloric density for all groups. Meal sizes in the low-cost group were similar to those of the no-cost group, but the latter took more meals per day and, therefore, consumed more total calories. High-cost meal patterns were parallel to those of the other groups, but with fewer, larger meals. All groups maintained a constant daily caloric intake across infusate concentrations. Meal patterns changed rapidly in response to these "covert" (untasted, isovolemic) changes in caloric density. Rats can, thus, integrate the cost of obtaining food and its postingestive benefits in the absence of mediating orosensory cues.

Effects of the lipase inhibitor orlistat on intake and preference for dietary fat in rats.

Orlistat (Ols), a potent inhibitor of pancreatic lipase, was added to the fat source (1 or 4 mg Ols/g fat) of a macronutrient self-selection diet fed to adult female rats. The rats responded to the drug-induced reduction in fat absorption by decreasing their dietary fat intake and increasing their protein and carbohydrate intake in a dose-related manner. Total caloric intake also increased, but body weight gain was inhibited compared with the nondrug control group. When Ols was removed from the diet, nutrient selection, caloric intake, and body weight returned to control levels. In additional short-term experiments (30 min/day), rats developed a preference for a plain fat diet over an Ols-fat diet (4 mg/g fat) and also for a cue flavor paired with plain fat over a flavor paired with Ols-fat. Yet, when not given the choice, the rats consumed nearly as much Ols-fat as plain fat diet. These results indicate that, by reducing fat absorption, Ols reduced the attractiveness of dietary fat, although it did not make the fat diet aversive. In clinical use, lipase inhibitors may be effective in reducing dietary fat intake by reducing both the consumption and absorption of fat.

The importance of taste and palatability in carbohydrate-induced overeating in rats.

Rats offered a carbohydrate solution (sugar or polysaccharide) in addition to chow typically overeat and gain excessive weight. The present study sought to determine if the palatable taste of these solutions contributes to the overeating response. Adult female rats were fitted with chronic intragastric catheters and given ad libitum access to chow and a drinking fluid that was paired with intragastric infusions. For one group (P + S), the flavored solution was a highly preferred mixture of 2% Polycose and 0.2% saccharin paired with intragastric infusions of 30% Polycose. For a second group (SOA), the flavored solution (0.03% sucrose octaacetate) had unpreferred bitter taste and was paired with intragastric infusions of 32% Polycose. Thus both groups were effectively exposed to the postingestive effects of 32% Polycose but paired with a palatable (P + S) or unpalatable (SOA) flavor. A control group had water to drink paired with intragastric water infusions. During the 4-wk experimental period, the P + S group consumed 34% more total energy (chow + Polycose) and gained more weight than did the SOA and control groups. The P + S group also consumed substantially more flavored solution and more energy as Polycose compared with the SOA group. The SOA group did not gain reliably more weight than the control group, although their total energy intake was 13% higher than that of the controls. In a choice test conducted at the end of the experiment, the P + S group displayed a strong preference (98%) for the Polycose + saccharin solution over water. In contrast to the control group, which avoided the SOA solution relative to water, the SOA group preferred the SOA solution (71%) confirming prior reports of Polycose-conditioned SOA preferences. These results demonstrate that palatability has a major effect on the overeating and obesity produced by carbohydrate solutions. In the absence of a highly preferred taste, the postingestive actions of Polycose produce only a small hyperphagic effect and no excess weight gain.

Carbohydrate- and protein-conditioned flavor preferences: effects of nutrient preloads.

Food-deprived rats were trained to associate one flavor (CSProt) with intragastric (IG) infusions of protein (PROT; 10% calcium caseinate), a second flavor (CSCHO) with IG infusions of carbohydrate (CHO; 10% protein), and a third flavor (CS-) with IG water infusions during 30 min/day training sessions. (The CS flavors were cherry, grape, and orange saccharin solutions.) In subsequent two-bottle tests the rats reliably preferred both the CSProt 2nd CSCHO to the CS- and equally preferred the CSProt and CSCHO. The preference for the two nutrient-paired flavors was not altered by IG preloads of PROT or CHO delivered as three loads 120, 40, and 5 min prior to testing. However, single oral + gastric preloads of CSCHO + IG CHO and CSProt + IG PROT 45 min prior to test selectively increased the preference for the CSProt and CSCHO, respectively. In subsequent gastric-only and oral-only tests single IG preloads of PROT and CHO, but not CSProt and CSCHO preloads, selectively altered the rats' preference for CSCHO vs. CSProt. In a second experiment with new rats, oral + gastric preloads again selectively altered the preference for the CSCHO vs. CSProt, but gastric-only preloads failed to have this effect. These results demonstrate that rats can learn to associate different flavors with the postingestive effects of different nutrients, and modify their flavor preferences after nutrient preloads. Oral + gastric preloads were most effective in altering flavor preferences, whereas gastric-only preloads had inconsistent effects and oral-only preloads were ineffective.

Nutrient-conditioned flavor preference and acceptance in rats: effects of deprivation state and nonreinforcement.

Rats acquire strong preferences for flavors paired with intragastric (IG) Polycose infusions. The present study investigated the ingestive responses to these flavors when they were no longer reinforced. All rats were first trained to associate a CS+ flavor with IG 32% Polycose and a CS- flavor with IG water during 23 h/day sessions. Experiment 1 examined the effects of deprivation on short-term (4 h/day) CS intakes under nonreinforcement conditions. Food deprivation selectively enhanced CS+ intake whereas water deprivation increased both CS+ and CS- intakes. The results also suggested that extinction training differentially affects CS+ preference and acceptance as measured by relative and absolute intakes. This was confirmed in a second experiment. The rats' absolute intake of the CS+ solution during 4 h/day sessions significantly decreased when it was no longer paired with IG Polycose infusions; yet the rats continued to show a strong preference for the CS+ over the CS- in both 4 h and 23 h/day tests. Experiment 3 examined the effects of extensive long-term extinction training on the CS+ preference. When the alternative was plain water, the CS+ preference extinguished over 12 days. Yet the same rats continued to prefer the CS+ to the CS- during 12 additional extinction days and during 16 extinction days that followed a 1-month rest period. The CS+ preference relative to the CS- was blocked only when the CS+ was paired with food deprivation and the CS- was paired with food repletion. These experiments confirm earlier reports of the resistance of Polycose-conditioned flavor preferences to extinction but also show that flavor acceptance is more susceptible to extinction. Flavor acceptance was not completely extinguished, however, and remained responsive to deprivation state.

Learned preferences for real-fed and sham-fed polycose in rats: interaction of taste, postingestive reinforcement, and satiety.

Nutrient-conditioned flavor preferences have been obtained with a variety of procedures, but one study indicates that rats do not readily learn to prefer the flavor of real-fed food to the flavor of sham-fed food. This issue was reexamined in food-deprived rats trained to real feed (gastric fistula closed) and sham feed (gastric fistula open) different flavored Polycose solutions (cherry or grape). Sham-feeding intakes in the one-bottle training sessions (30 min) were limited to the amount consumed in real-feeding sessions; intakes were not limited during two-bottle preference tests (10 min). In Experiment 1, when tested with 32% Polycose solutions, the animals tended to prefer the sham-fed Polycose to the real-fed Polycose. When subsequently trained and tested with 8% Polycose solutions, the same animals displayed a strong preference (approximately 90%) for the real-fed Polycose. These findings were confirmed in Experiment 2 using separate groups. Rats trained with 8% Polycose readily learned to prefer the real-fed solution, and rats trained with 32% Polycose initially preferred the sham-fed Polycose solution. With additional training the 32% Polycose group developed a preference for the real-fed solution. In experiment 3, rats trained to real feed flavored 8% and 32% Polycose solutions developed strong preferences for the 32% solution. This finding suggests that the tendency to prefer sham-fed to real-fed 32% Polycose is not due to aversive postingestive effects of the real-fed solution. Taken together, the results indicate that Polycose has a postingestive positive reinforcing effect that can be revealed using the sham-feeding preparation.(ABSTRACT TRUNCATED AT 250 WORDS)

Glucose- and fructose-conditioned flavor preferences in rats: taste versus postingestive conditioning.

Flavor preferences conditioned by glucose and fructose were compared using two training methods. With the simultaneous method preferences can be reinforced by the flavor and/or the postingestive consequences of nutrients, whereas with the delayed method preferences are reinforced only by postingestive nutritive effects. In Experiment 1, food-deprived rats displayed similar preferences for flavors (CS+) added to an 8% glucose or 8% fructose solution over flavors (CS-) added to a noncaloric saccharin solution (simultaneous conditioning). Other rats learned to prefer a CS+ flavor paired with the delayed (10 min) presentation of 8% glucose over a CS- flavor paired with delayed saccharin. Fructose failed to condition a flavor preference with the delayed paradigm. Taken together, these data suggest that the preference for a flavor mixed in a fructose solution is reinforced by the sweet taste, not the postingestive effects of the sugar. Experiment 2 tested this idea by devaluing the taste of the sugar solutions by quinine adulteration. Rats initially avoided both glucose-quinine and fructose-quinine solutions in favor of a saccharin solution. Following one-bottle training, they came to prefer the glucose-quinine but not the fructose-quinine solution over the saccharin solution. The glucose-trained rats also showed stronger preferences for sucrose-quinine solutions than did the fructose-trained rats. These findings, along with other recent data, indicate that fructose-conditioned preferences are based primarily on the sugar's palatable taste. Glucose, in contrast, can condition strong preferences based on its taste as well as its postingestive actions.

Flavor preferences conditioned by intragastric infusions of dilute polycose solutions.

Prior studies have obtained strong conditioned preferences (> 90%) for flavors paired with intragastric (IG) infusions of Polycose (hydrolyzed starch) over different flavors paired with IG water infusions. These experiments used concentrated Polycose solutions (16-32%; 0.6-1.2 kcal/ml) that contributed 50% or more of the animals' daily caloric intake. The present experiment sought to determine if calorically dilute solutions would also support preference conditioning. Adult female rats with gastric cannulas were trained with a cue solution (the CS+) paired with Polycose infusions. On alternate days a different cue solution (the CS-) was paired with IG water infusions. The CSs were unsweetened or saccharin-sweetened citric acid and sucrose octaacetate solutions that were available 22 h/day; chow was freely available. Separate groups were infused with Polycose at concentrations of 0.5%, 1%, 2%, and 4% (0.019-0.152 kcal/ml). The three higher concentrations conditioned flavor preferences of up to 89%. The infused Polycose represented only 1-14% of the animals' daily caloric intake and did not reliably reduce chow consumption. These findings demonstrate the sensitivity of the viscerosensory feedback system mediating flavor conditioning.

The rat's preference for sucrose, polycose and their mixtures.

In one-bottle tests (30 min/day) food-deprived rats consumed significantly more 2% Polycose than 2% sucrose, and yet in two-bottle choice tests they did not consistently prefer Polycose to sucrose. A previously proposed explanation for this effect is that rats habituate, or develop oral satiety, to the flavor of sucrose more rapidly than to Polycose, and sucrose satiety generalizes to Polycose. This idea was not supported by the results of sequential 15-min one-bottle tests: the intake of 2% sucrose did not suppress subsequent Polycose intake. Instead, the failure of rats to consistently prefer Polycose to sucrose was attributed to the development of a drinking pattern that allowed the rats to mix the two solutions in their mouths. When the drinking spouts were nearby, the rats consumed similar amounts of sucrose and Polycose, but when the spouts were far apart, they consumed more Polycose than sucrose. That rats prefer the combined flavor of sucrose and Polycose to either flavor alone was confirmed in subsequent tests. Rats strongly preferred a 1% sucrose + 1% Polycose mixture to a plain 2% sucrose or 2% Polycose solution. They also preferred sucrose-Polycose mixtures to plain solutions at concentrations ranging up to 32%. The potency of the sucrose-Polycose mixture is presumably related to the activation of separate "sweet" and "polysaccharide" taste channels in the rat.

Flavor preference produced by intragastric polycose infusions in rats using a concurrent conditioning procedure.

Prior studies demonstrate that nondeprived rats learn to prefer a flavor (the CS+) paired with intragastric (IG) nutrient infusions over another flavor (the CS-) paired with IG water infusions when the flavors are presented on alternate days. The present experiment determined if a nutrient-based preference could be established when both flavors were concurrently available. Adult female rats were fitted with two chronic gastric catheters and were given ad lib access to chow and two flavored solutions (a bitter sucrose octaacetate solution and a sour citric acid solution). Using an automated infusion system, consumption of the CS+ flavor was paired with IG infusions of 32% Polycose, and consumption of the CS- flavor was paired with IG infusions of water. Seven of the eight rats tested developed a strong flavor preference for the CS+ over the CS-; their CS+ preference increased from 53% on day 1 to 95% on day 8. (The remaining animal eventually displayed a 97% CS+ preference after it received one-bottle training with the CS flavors.) During the first several days of training, the rats tended to drink primarily one flavor per day, and consumed primarily one flavor per drinking bout. This provided them with relatively discrete flavor-consequence experiences that presumably facilitated flavor preference conditioning.

Intragastric glucose but not fructose conditions robust flavor preferences in rats.

Prior work indicates that glucose and fructose differ in their postingestive reinforcing effects. The present study investigated this phenomenon by training rats to associate the intake of flavored water with intragastric (IG) infusions of 16% sugar solutions. In experiment 1, rats had one flavor [conditional stimulus (CS)] paired with IG sugar infusions (CS+; e.g., cherry) and another flavor paired with IG water (CS-; e.g., grape) 23 h/day; Chow was available ad libitum. In subsequent choice tests, rats infused with glucose displayed a strong preference (89%) for the CS+ flavor, whereas rats infused with fructose showed only a small and nonreliable CS+ flavor preference (62%). When next trained to associate one flavor (e.g., orange) with IG glucose and another flavor (e.g., strawberry) with IG fructose, rats in both groups developed a significant preference (81%) for the glucose-paired flavor. In experiment 2, food-deprived rats were trained 2 h/day to associate a CS+ flavor with IG sugar and a CS- flavor with IG water infusions. The glucose-reinforced rats displayed a near-total preference (95%) for the CS+ flavor, whereas fructose-reinforced rats showed a much smaller CS+ preference (67%). The preference findings indicate that the postingestive consequences of glucose are much more reinforcing than those of fructose. It appears that food conditioning is mediated by chemospecific actions of nutrients rather than their general satiating or energy repleting effects.