The CS-US delay gradient in flavor preference conditioning with intragastric carbohydrate infusions

The nature of training in flavor preference learning determines the underlying associative structure

Pairing a palatable flavor (US) with an initially neutral flavor cue (CS) results in an acquired conditioned preference for the latter. Two main associations have been proposed to explain the acquisition of flavor preferences: Flavor-Flavor and Flavor-Nutrient learning. Although the hedonic reaction triggered by US consumption has also been suggested as a possible additional component underlying acquired flavor preference, this issue has received little attention. Here we explored whether the amount of training to the CS-US compound can favor the formation of a Flavor-Hedonic reaction association using rats as subjects and sucrose as the US. We expected that the more exposure to the CS-US compound, the stronger the S-R type association. Since S-R associations are not sensitive to devaluation procedures, we used a Sensory-Specific Satiety procedure to devalue the US after conditioning and then measured preferences for the CS. On Experiment 1 with a short restrictive training (classic procedure), preference for the CS was decreased after devaluation of the US compared to the control condition. On Experiment 2, with short unrestrictive training, preference for the CS was again weakened. Experiment 3 with a long unrestrictive training, rats expressed preference for the CS regardless of the devaluation procedure. These results suggest that, as with an instrumental paradigm, extensive training in flavor preference learning undermines the US devaluation effect.

Does drinking saccharin weaken an association of sweet with calories? Pre-exposure effects in flavor preference learning

The main aim of this experiment was to examine the claim that exposure to non-nutritive sweeteners weakens the formation of a sweet-calorie association. Three groups of food-deprived rats received training in which they drank an almond-flavored maltodextrin and saccharin solution. A final test phase assessed their preference for almond. The groups differed in preexposure prior to training. One was pre-exposed to saccharin, one to saccharin plus maltodextrin, and the third, control condition, received only water at this stage. When the rats continued under food deprivation for the test phase, the group exposed to the compound (saccharin plus maltodextrin) showed a weaker preference than the other two groups, while those pre-exposed to saccharin showed as strong a preference as the controls. When the test was conducted with the rats no longer food-deprived, only the water group showed a strong preference. These results support the proposal that rats can form both flavor-flavor and flavor-nutrient associations, expression of which will depend on motivational state. They did not find support for the suggestion that prior exposure to a non-nutritive sweetener can enhance subsequent learning about the nutritive properties of a sweet food.

Another look at the extinction of conditioned flavor preferences: Amount of training and tests for spontaneous recovery

A conditioned flavor preference develops when hungry or thirsty rats experience a neutral flavor mixed in solution with a nutrient. In two sets of studies, we previously demonstrated that this learned preference is highly sensitive to flavor nonreinforcement (i.e., exposure to the flavor without the nutrient) either prior to (latent inhibition), during (partial reinforcement), or following (extinction) flavor-nutrient pairings. In each of these studies we employed a nutrient devaluation procedure to assess the integrity of specific flavor-nutrient associations following extinction, but more recently Gonzalez, Morillas, and Hall (Journal of Experimental Psychology: Animal Learning & Cognition, 42, 380-390, 2016) observed that sensitivity to extinction in thirsty rats in this preparation may depend upon use of a post-conditioning nutrient devaluation procedure. To assess the generality of our earlier results, but without including a post-conditioning nutrient devaluation phase, we assessed in three experiments the role of the number of flavor-nutrient pairings given prior to extinction and the possibility of spontaneous recovery following a 3-week delay. We observed that extinction consistently weakened the flavor preference in thirsty rats (in spite of the absence of a nutrient devaluation procedure) and also found no evidence for spontaneous recovery. These results establish that our prior findings that conditioned flavor preferences are weakened by extinction are quite robust in thirsty rats and that these extinction effects may be fairly permanent.

Nutrient-conditioned intake stimulation does not require a distinctive flavor cue in rats

The postoral actions of nutrients in rodents can stimulate intake and condition flavor preferences through an appetition process. Appetition is revealed in rodents by their increased intake of and preference for a flavored solution paired with intragastric (IG) nutrient infusions. Here we determined if IG 16% maltodextrin (MD) infusions can stimulate intake and preference in the absence of a distinctive flavor cue. Rats implanted with IG catheters were given chow and water 2 h/day followed, 2 h later, by 20-h oral access to water paired with IG MD infusions. Other rats were given bitter sucrose octaacetate solution (SOA) paired with IG MD infusions 20 h/day. Over 8 test days, the SOA rats increased their total 20-h fluid intake (oral + IG) from 26 to 119 g/20 h and Water rats increased their intake from 31 to 96 g/20 h. When infused IG with water instead of MD in a 4-day extinction test, the SOA and Water groups reduced their fluid intakes to 45-48 g/20 h. When oral fluids were again paired with IG MD infusions, the SOA and Water groups increased their intakes to 115 and 109 g/20 h, respectively. In two-bottle tests, the SOA rats drank more SOA paired with IG MD than water paired with IG water. Water rats given the choice of a water bottle paired with IG MD and water bottle paired with IG water did not consistently prefer the H2O/ID MD bottle. Instead they displayed side or sipper tube preferences although neither cue was consistently paired with IG MD during one-bottle training.

Exposures to conditioned flavours with different hedonic values induce contrasted behavioural and brain responses in pigs

This study investigated the behavioural and brain responses towards conditioned flavours with different hedonic values in juvenile pigs. Twelve 30-kg pigs were given four three-day conditioning sessions: they received three different flavoured meals paired with intraduodenal (i.d.) infusions of 15% glucose (F(Glu)), lithium chloride (F(LiCl)), or saline (control treatment, F(NaCl)). One and five weeks later, the animals were subjected to three two-choice feeding tests without reinforcement to check the acquisition of a conditioned flavour preference or aversion. In between, the anaesthetised pigs were subjected to three (18)FDG PET brain imaging coupled with an olfactogustatory stimulation with the conditioned flavours. During conditioning, the pigs spent more time lying inactive, and investigated their environment less after the F(LiCl) than the F(NaCl) or F(Glu) meals. During the two-choice tests performed one and five weeks later, the F(NaCl) and F(Glu) foods were significantly preferred over the F(LICl) food even in the absence of i.d. infusions. Surprisingly, the F(NaCl) food was also preferred over the F(Glu) food during the first test only, suggesting that, while LiCl i.d. infusions led to a strong flavour aversion, glucose infusions failed to induce flavour preference. As for brain imaging results, exposure to aversive or less preferred flavours triggered global deactivation of the prefrontal cortex, specific activation of the posterior cingulate cortex, as well as asymmetric brain responses in the basal nuclei and the temporal gyrus. In conclusion, postingestive visceral stimuli can modulate the flavour/food hedonism and further feeding choices. Exposure to flavours with different hedonic values induced metabolism differences in neural circuits known to be involved in humans in the characterization of food palatability, feeding motivation, reward expectation, and more generally in the regulation of food intake.

Role of gut nutrient sensing in stimulating appetite and conditioning food preferences

The discovery of taste and nutrient receptors (chemosensors) in the gut has led to intensive research on their functions. Whereas oral sugar, fat, and umami taste receptors stimulate nutrient appetite, these and other chemosensors in the gut have been linked to digestive, metabolic, and satiating effects that influence nutrient utilization and inhibit appetite. Gut chemosensors may have an additional function as well: to provide positive feedback signals that condition food preferences and stimulate appetite. The postoral stimulatory actions of nutrients are documented by flavor preference conditioning and appetite stimulation produced by gastric and intestinal infusions of carbohydrate, fat, and protein. Recent findings suggest an upper intestinal site of action, although postabsorptive nutrient actions may contribute to flavor preference learning. The gut chemosensors that generate nutrient conditioning signals remain to be identified; some have been excluded, including sweet (T1R3) and fatty acid (CD36) sensors. The gut-brain signaling pathways (neural, hormonal) are incompletely understood, although vagal afferents are implicated in glutamate conditioning but not carbohydrate or fat conditioning. Brain dopamine reward systems are involved in postoral carbohydrate and fat conditioning but less is known about the reward systems mediating protein/glutamate conditioning. Continued research on the postoral stimulatory actions of nutrients may enhance our understanding of human food preference learning.

Rapid post-oral stimulation of intake and flavor conditioning by glucose and fat in the mouse

Although widely assumed to have only satiating actions, nutrients in the gut can also condition increases in intake in some cases. Here we studied the time course of post-oral nutrient stimulation of ingestion in food-restricted C57BL/6J mice. In experiment 1, mice adapted to drink a 0.8% sucralose solution 1 h/day, rapidly increased their rate of licking (within 4-6 min) when first tested with an 8% glucose solution and even more so in tests 2 and 3. Other mice decreased their licking rate when switched from sucralose to 8% fructose, a sugar that is sweet like glucose but lacks positive post-oral effects in mice. The glucose-stimulated drinking is due to the sugar's post-oral rather than taste properties, because sucralose is highly preferred to glucose and fructose in brief choice tests. A second experiment showed that the glucose-stimulated ingestion is associated with a conditioned flavor preference in both intact and capsaicin-treated mice. This indicates that the post-oral stimulatory action of glucose is not mediated by capsaicin-sensitive visceral afferents. In experiment 3, mice consumed flavored saccharin solutions as they self-infused water or glucose via an intragastric (IG) catheter. The glucose self-infusion stimulated ingestion within 13-15 min in test 1 and produced a conditioned increase in licking that was apparent in the initial minute of tests 2 and 3. Experiment 4 revealed that IG self-infusions of a fat emulsion also resulted in post-oral stimulation of licking in test 1 and conditioned increases in tests 2 and 3. These findings indicate that glucose and fat can generate stimulatory post-oral signals early in a feeding session that increase ongoing ingestion and condition increases in flavor acceptance and preference revealed in subsequent feeding sessions. The test procedures developed here can be used to investigate the peripheral and central processes involved in stimulation of intake by post-oral nutrients.