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)

Robust conditioned flavor preference produced by intragastric starch infusions in rats.

Rats were intragastrically infused with hydrolyzed starch (16% Polycose) or water as they drank cherry- or grape-flavored water during 23-h/day tests; chow was available ad libitum. After 4 conditioning days the rats displayed a near-total preference (96%) for the starch-paired flavor over the water-paired flavor in two-choice tests. This conditioned flavor preference persisted during a 4-day extinction test when both flavors were paired with water infusions. The results demonstrate that the postingestive actions of starch are rewarding to nondeprived rats and can condition strong and long-lasting flavor preferences.

A comparison of the effects of atropine on real-feeding and sham-feeding of sucrose in rats.

In Experiment 1 the influence of atropine methyl nitrate on the sham-feeding response of adult female rats to a sucrose solution was determined. Atropine (1 or 5 mg/kg) reliably suppressed the sham-intake of sucrose when the drug was administered 30 or 0 min prior to, or 17 min after the start of the feeding session. The suppressive effect was less, however, when the drug was administered 30 min before testing compared to the other two injection-test conditions. In Experiment 2 atropine failed to reliably decrease the real-feeding of a sucrose solution whether it was injected 30, 15, or 0 min prior to testing. These results were replicated in Experiment 3; atropine (0 min injection-test interval) reduced the sham-intake but not the real-intake of a sucrose solution. However, atropine decreased the rate of feeding under both real- and sham-feeding conditions. The fact that atropine reduced feeding rate but not meal size in the real-feeding condition was attributed to the drug's lack of effect on postingestive satiety. The present findings along with other recent results indicate that (1) the injection-test interval is a potentially important variable in studies involving atropine; (2) results obtained with sham-feeding animals do not always generalize to real-feeding animals; and (3) cholinergically-mediated cephalic responses are of questionable importance in the control of meal size.

Oral versus postingestive origin of polysaccharide appetite in the rat.

Previous studies have revealed that rats consume substantial amounts of polysaccharide solutions, even if the solutions are made bitter with the addition of sucrose octa acetate (SOA). The present experiment used the gastric sham-feeding preparation to determine if it is the orosensory or postingestive properties of polysaccharides that motivate rats to consume polysaccharide (Polycose) solutions. In Experiment 1, food deprived rats sham fed less of a 0.05% SOA + 32% Polycose solution than they did of a 32% glucose solution, but their SOA-Polycose intake was still considerable (44 ml/hr). The same rats refused to sham feed SOA-gum and SOA-sugar solutions that were similar to the SOA-Polycose solution in bitter taste, viscosity and free sugar content. In Experiment 2, rats sham fed as much of a 32% Polycose solution as they did of a 32% sucrose solution. Despite the gastric fistula, some of the ingested Polycose was absorbed as evidenced by an increase in the rats' blood glucose levels. The addition of acarbose, a drug that inhibits polysaccharide digestion, to the Polycose solution blocked the increase in blood glucose, but did not reduce the rats' sham feeding of the solution. These findings indicate that it is the orosensory (presumably taste) properties of polysaccharide solutions, not their postingestive effects, that initially attract rats to the solutions. The results question the assumption that polysaccharides are "tasteless" to animals.

Taste preference thresholds for Polycose, maltose, and sucrose in rats.

The taste preference thresholds of adult female rats for polysaccharide (Polycose), maltose, and sucrose were compared. The nondeprived animals were given 24-hr two-bottle preference tests (saccharide solution vs. water) and, starting at 0.008%, the saccharide concentration was increased daily. The rats first preferred the Polycose solution to water at 0.01% (0.0001 M), the maltose solution to water at 0.09% (0.0025 M), and the sucrose solution to water at 0.09% (0.0026 M). Thus, on a molar basis the rats' Polycose threshold was 25 to 26 times lower than their maltose and sucrose threshold. It was postulated that the low taste threshold for polysaccharides allows the rat to detect starch which, unlike sugar, is very low in solubility.

Qualitative differences in polysaccharide and sugar tastes in the rat: a two-carbohydrate taste model.

A conditioned taste aversion paradigm was used to assess the qualitative similarities between the tastes of a polysaccharide (Polycose) solution and sugar solutions (sucrose, maltose, glucose, fructose). In Experiment I, three groups of female rats were water deprived and conditioned to avoid a 0.025 M Polycose, a 0.1 M sucrose, or a 0.1 M maltose solution by pairing solution consumption with a lithium chloride (LiCl) injection; in a control group water consumption was paired with the LiCl injection. The extent to which the experimental groups generalized their conditioned aversion to the other three solutions was then assessed. The Polycose-conditioned group avoided the maltose solution more than the sucrose solution, and the maltose-conditioned group avoided the Polycose solution more than the sucrose solution. The sucrose-conditioned group avoided the maltose and Polycose solutions to the same relatively low degree. In additional tests the three experimental groups showed similar aversions to a glucose solution, but only the sucrose-conditioned rats avoided a fructose solution. Rats in a second experiment also displayed relatively little cross-generalization between Polycose and sucrose aversions even though they were tested with different solution concentrations. Additional tests confirmed the results obtained in Experiment 1 with maltose, glucose, and fructose solutions, and also revealed that the sucrose-conditioned group, but not the Polycose-conditioned group avoided saccharin solutions. Neither Polycose- nor sucrose-conditioned groups avoided quinine, sodium chloride, or hydrochloric acid solutions. These results, along with other recent findings, suggest that rats have two types of "carbohydrate" taste receptors, one for polysaccharides and one for sucrose, which produce qualitatively distinct gustatory sensations.

Sham-feeding response of rats to Polycose and sucrose.

Adult female rats were fitted with gastric fistulas and maintained at 85% of their ad lib body weight. Their real-feeding (fistula closed) and sham-feeding (fistula open) responses to polysaccharide (Polycose) and sucrose solutions were measured during 30 min/day one-bottle tests. The rats consumed similar amounts of a 1% Polycose solution during real- and sham-feeding tests, but their sham-intakes of 4%, 16% and 32% Polycose solutions greatly exceeded their real-intakes of these solutions. Similar results were obtained with sucrose solutions. The rats sham-fed more Polycose than sucrose at the 1% and 4% concentrations, while their sham-intakes of the 16% and 32% Polycose and sucrose solutions were comparable. In subsequent two-solution sham-feeding tests, the rats preferred 1% Polycose to 1% sucrose, but preferred sucrose to Polycose at 4%, 16% and 32% concentrations. These preference results indicate that rats find Polycose more palatable than sucrose at low concentrations, but sucrose more palatable at high concentrations. In addition, the findings that the rats preferred 4% sucrose to 4% Polycose in the two-bottle test, but sham-fed more 4% Polycose than 4% sucrose in the one-bottle tests, suggest that sucrose is more "orally-satiating" than is Polycose. These results provide further evidence for qualitative differences in the tastes of sucrose and polysaccharide. They also indicate that the amount of solution sham-fed does not necessarily reflect the palatability of the solution.

Influence of saccharin on Polycose, sucrose, and glucose intake and preference in rats.

The effect of adding 0.125% saccharin to 3% or 32% solutions of Polycose, sucrose and glucose on the fluid intake and preference of adult female rats was examined. In Experiment 1, the rats consumed more of a 3% Polycose + 0.125% saccharin solution (P + s) than of either a 3% Polycose or 0.125% saccharin solution; similar results were obtained with sucrose + saccharin (S + s) and glucose + saccharin (G + s) solutions. The polydipsic effects of the P + s, S + s, and G + s solutions were comparable (225 to 278 ml/day). Adding saccharin to 32% Polycose, sucrose, or glucose solutions did not increase solution intake. In two-solution preference tests, though, the rats preferred the 32% Polycose + saccharin and 32% glucose + saccharin solutions to 32% Polycose and 32% glucose solutions, respectively. Saccharin did not reliably affect the preference for the 32% sucrose solution. In Experiment 2, the preference for 3% carbohydrate solutions was assessed using two-solution tests. The rats preferred 3% sucrcose to 3% Polycose or 3% glucose; they also preferred 3% Polycose to 3% glucose. When saccharin was added to the solutions, the rats displayed equal preferences for the S + s and P + s solutions, and for the P + s and G + s solutions but they strongly preferred the S + s to the G + s solution. Recent findings suggest that polysaccharides such as Polycose taste qualitatively different from sucrose and saccharin to rats, i.e., have a "nonsweet" taste.(ABSTRACT TRUNCATED AT 250 WORDS)

Starch preference in rats.

Several experiments examined the preference of adult female rats for starch and starch-derived polysaccnarides using short- and long-term two-choice tests. In Experiment 1, food-deprived rats displayed an immediate preference for powdered corn starch over powdered cellulose in 30-min/day tests. This supports the hypothesis that rats have starch (polysaccharide) taste receptors. The rats also preferred the starch powder to Polycose powder. This preference was unexpected since Polycose, being partially hydrolyzed corn starch, should be a more effective taste stimulus than corn starch. Experiment 2 revealed that nondeprived rats also preferred starch to Polycose in 30-min/day tests, but reversed their preference in 24-hr/day tests; the preference reversal was attributed to postingestive factors. Additional experiments demonstrated that rats preferred branched-chain starch (amylopectin) to unbranched starch (amylose) in both short- and long-term tests. Rats also preferred amylopectin to Polycose in short-term tests when the saccharides were in powder form, but preferred Polycose to amylopectin when the saccharides were in liquid or gel form. The preference for amylopectin powder was not due to the noncarbohydrate constituents (fatty acids, proteins, others) in the starch since rats also preferred chemically purified amylopectin to Polycose powder. Additional tests suggested that texture differences do not explain the rats' preference for starch powder over Polycose powder. Taken together, the results demonstrate that rats have a robust preference for starch but the orosensory determinants of this preference are not completely understood.

On the role of the mouth and gut in the control of saccharin and sugar intake: a reexamination of the sham-feeding preparation.

Adult female rats, each fitted with a gastric fistula, were tested for their "normal-feeding" (fistula closed) and "sham-feeding" (fistula open) response to saccharin and sugar solutions under a variety of conditions. When hungry, rats consumed no more of a 0.2% saccharin solution with their fistula open than they did with their fistula closed. Increasing or decreasing the saccharin concentration did not increase the amount of solution sham fed, but adding a small amount of glucose (3%) to the saccharin solution did increase the amount sham fed. Thirsty rats, unlike hungry, significantly increased their 0.2% saccharin solution intake when tested with an open fistula. When tested with a 32% glucose solution, hungry rats consumed up to six times more solution with their fistula open than with their fistula closed. The hungry rats also sham fed significantly more of the 32% glucose solution than of the 0.2% saccharin solution or 0.2% saccharin + 3% glucose solution. Sham-feeding of a 32% sucrose solution significantly elevated blood glucose levels, but blocking this effect by adding acarbose, a drug that inhibits sucrose digestion, did not reduce the amount of solution sham fed. Several possible explanations for the differential sham-feeding response to saccharin and sugar solutions are discussed.

Is gastric sham feeding really sham feeding?

Rats were fitted with gastric cannulas, food deprived, and allowed to drink a sugar solution that drained out of the opened cannula; i.e., the rats sham-fed. Although this procedure is thought to prevent absorption of ingested food, it was found that the sham feeding of a 32% glucose or sucrose solution significantly elevated blood glucose levels. The addition of acarbose, a drug that inhibits the digestion of sucrose, to the 32% sucrose solution blocked the blood glucose rise, as did closing the pylorus with an inflatable pyloric cuff. Neither the drug nor the cuff, however, reduced the amount of sucrose solution consumed. These findings indicate that gastric sham feeding does not necessarily prevent the digestion and absorption of food, although absorption is not essential for the appearance of a vigorous sham-feeding response. Nevertheless the possibility that neural or hormonal feedback from the stomach contributes to the sham-feeding response cannot be excluded, and until this issue is resolved the results of gastric sham-feeding studies should be interpreted with caution.