Post-oral infusion sites that support glucose-conditioned flavor preferences in rats.

Rats learn to prefer a flavored solution (CS+) paired with a gastrointestinal glucose infusion over an alternate flavor (CS-) paired with a non-caloric infusion. Prior work implicates a post-gastric site of glucose action, which is the focus of this study. In Exp. 1, male rats (8-10/group) were infused in the duodenum (ID), mid-jejunum (IJ), or distal ileum (II) with 8% glucose or water as they drank saccharin-sweetened CS+ and CS- solutions, respectively, in one-bottle 30-min sessions. Two-bottle tests (no infusions) were followed by a second train-test cycle. By the second test, the ID and IJ groups preferred the CS+ (69%, 67%) to the CS- but the II group did not (48%). Satiation tests showed that ID and IJ infusions of glucose reduced intake of a palatable solution similarly, while II infusions were ineffective. In Exp. 2, rats (10/group) drank CS solutions in one-bottle, 30-min sessions and were given 2-h ID or hepatic portal vein (HP) infusions. The CS+ and CS- were paired with 10 ml infusions of 10% glucose and 0.9% saline, respectively. Following 8 training sessions, the ID group preferred the CS+ (67%) to the CS- but the HP group did not (47%) in a two-bottle test. The similar CS+ preferences displayed by ID and IJ, but not II groups implicate the jejunum as a critical site for glucose-conditioned preferences. A pre-absorptive glucose action is indicated by the CS+ preference displayed by ID but not HP rats in Exp. 2. Our data were obtained with non-nutritive CS solutions. HP glucose infusions are reported to condition preferences for a flavored food that itself has pre- and post-absorptive actions. Thus, there may be multiple sites for glucose conditioning with the upper or mid-intestines being the first site of action.

Rapid acquisition of conditioned flavor preferences in rats.

Rats learn to prefer flavors paired with the post-oral effects of glucose. The present study examined how rapidly they acquire this preference. In Experiment 1, food-restricted rats were given repeated three-session training/testing cycles: one 30-min session with a CS+ flavor paired with intragastric (IG) infusion of 16% glucose, another session with a CS- flavor paired with IG water, and a third session with a choice between the flavors with their infusates. The rats preferred the CS+ (69%) in the first choice session, and preference increased across the six cycles to 86%. These data demonstrate that the post-oral reinforcing action of glucose is potent enough to support one-trial learning. In Experiment 2, two groups of rats were trained in the same way, with the CS+ flavor paired with IG infusion of 16% glucose or 7.1% corn oil emulsion, but tests were conducted under extinction conditions, with both CS+ and CS- flavors paired with IG water. Significant preference for the CS+ was acquired more rapidly with glucose (71% CS+ in test 1) than with oil (69% CS+ in test 4). Consistent with previous work, the post-oral stimulation by glucose was more potent than that of isocaloric oil emulsion in conditioning preferences. The last experiment examined the acquisition rate for a flavor-taste conditioned preference. Rats were trained with a CS+ flavor mixed into an 8% fructose + 0.2% saccharin solution and a CS- flavor in 0.2% saccharin. The same three-session training/testing cycles were used, and in the tests the flavors were presented in saccharin. A significant 74% preference for the CS+ flavor was apparent by the second test. Together these studies show that the acquisition of flavor preferences, whether based on flavor-taste or flavor-nutrient associations, can be quite rapid.

Intragastric infusion of denatonium conditions flavor aversions and delays gastric emptying in rodents.

Because most naturally occurring toxins taste bitter to humans, any mechanism that reduces the rate at which bitter substances are ingested and digested should be adaptive. Based on the recent discovery of T2R bitter taste receptors in the gastrointestinal tract of rodents, we asked whether intragastric (IG) infusion of denatonium (a ligand for T2R receptors) would condition a flavor aversion and/or delay gastric emptying. Four experiments tested for post-oral responses to denatonium in rodents. First, Sprague-Dawley rats were trained to associate intake of a flavored solution (the CS+) with IG denatonium infusions, and intake of a different-flavored solution (the CS-) with IG water infusions during 30 min/day sessions. The rats acquired an aversion to the CS+ flavor when it was paired with IG infusions of 10 mM (but not 2.5 mM) denatonium. Intragastric infusions of 10 mM denatonium also delayed gastric emptying of food in the same rats. Second, we asked how long it took for rats to suppress their drinking while being infused IG with 10 mM denatonium. Rats drinking a palatable solution paired with IG infusions of 10 mM denatonium suppressed their licking within 6 min, as compared to rats infused IG with water. Third, we trained C57BL/6J (B6) mice 24 h/day to associate a CS+ flavor paired with IG infusions of 12 mM denatonium (diluted to 6 mM by orally consumed CS+). Like rats, the mice acquired a robust aversion to the CS+ flavor when it was paired with IG infusions of denatonium. A final experiment assessed the potential toxicity of denatonium. To this end, we gave B6 mice a 6 mM denatonium solution as their only source of water for 3 weeks. The mice grew normally and did not display any clinical signs of denatonium toxicosis. This study provides the first evidence that rodents respond to the presence of "bitter" substances in their gastrointestinal tract by generating both behavioral and physiological responses.

Obesity by choice revisited: effects of food availability, flavor variety and nutrient composition on energy intake.

Recent work suggested that the energy intake and weight gain of rats maintained on chow and 32% sucrose solution could be increased by simply offering more sources of sucrose [Tordoff M.G. Obesity by choice: the powerful influence of nutrient availability on nutrient intake. Am J Physiol 2002;282:R1536-R1539.]. In Experiment 1 this procedure was replicated but the effect was not: rats given one bottle of sucrose and five bottles of water consumed as much sucrose as those given five bottles of sucrose and one of water. Adding different flavors to the sucrose did not increase intakes further in Experiment 2. The relative potency of sucrose and other optional foods was studied in Experiment 3. Sucrose solution stimulated more overeating and weight gain than fat (vegetable shortening), and offering both sucrose and shortening did not generate further increases in energy intake. Finally, foods commonly used to produce overeating and weight gain were compared. Sucrose was less effective than a high-fat milk diet, and offering cookies in addition to the milk did not increase energy intake further. The nature of optional foods (nutrient composition and physical form) was markedly more important than the number of food sources available to the animals, and is a better contender as the reason for "obesity by choice".

Food deprivation enhances the expression but not acquisition of flavor acceptance conditioning in rats.

The postingestive actions of nutrients condition strong flavor preferences in rats and may also enhance flavor acceptance (increase total intake) in some situations. This study determined the impact of food deprivation on flavor preference and acceptance conditioned by intragastric (i.g.) infusions of glucose. Rats fitted with gastric catheters were trained (20 h/day) to associate a CS+ solution (bitter or sour) with i.g. 16% glucose and a CS- solution with water infusions. One group (FR) was food-restricted during the training sessions, while a second group (AL) was given food ad libitum. All rats were given 2-h access to food prior to the daily sessions. During one-bottle training, the FR rats consumed substantially more CS+ than CS- whereas AL rats drank only slightly more CS+ than CS-. In additional one-bottle acceptance tests, the FR and AL rats consumed substantially more CS+ than CS- when both groups were food-restricted, but only slightly more CS+ than CS- when both groups had food ad libitum. Throughout the experiment, the FR and AL rats displayed equally strong CS+ preferences in two-bottle choice tests irrespective of their deprivation state during the test. The findings indicate that food restriction stimulates the intake of a CS+ flavor that is (or was previously) paired with i.g. glucose infusions but does not fundamentally alter the learned association between the CS+ flavor and the post-oral nutrient stimulus.

Flavor preferences conditioned by intragastric nutrient infusions in food restricted and free-feeding rats.

The role of deprivation state in flavor preference conditioning by nutrients was investigated in rats fitted with intragastric (IG) catheters. In different experiments, food restricted (FR) and food ad libitum (AL) groups were trained to drink one flavored solution (CS+) paired with IG infusions of maltodextrin, corn oil, or casein and another flavored solution (CS-) paired with IG water infusions. Training intakes of the CS solutions were limited to equate the exposure of the FR and AL groups. The IG nutrient infusions conditioned flavor preferences in FR and AL groups which, in three of four experiments, were of similar magnitude. Food restriction did, however, increase the overall intake of the CS+ solutions during testing. Rats trained with one CS+ while food restricted and a second CS+ while food unrestricted showed similar preferences for the two CS+ flavors. Prefeeding AL rats to satiety with chow prior to daily training sessions did not prevent them from developing a preference for a CS+ paired with IG maltodextrin. These findings indicate that the postoral actions of nutrients are reinforcing in food sated as well as hungry rats.