Differential reinforcing and satiating effects of intragastric fat and carbohydrate infusions in rats

Highly processed foods can be considered addictive substances based on established scientific criteria

BACKGROUND

There is growing evidence that an addictive-eating phenotype may exist. There is significant debate regarding whether highly processed foods (HPFs; foods with refined carbohydrates and/or added fats) are addictive. The lack of scientifically grounded criteria to evaluate the addictive nature of HPFs has hindered the resolution of this debate.

ANALYSIS

The most recent scientific debate regarding a substance's addictive potential centered around tobacco. In 1988, the Surgeon General issued a report identifying tobacco products as addictive based on three primary scientific criteria: their ability to (1) cause highly controlled or compulsive use, (2) cause psychoactive (i.e. mood-altering) effects via their effect on the brain and (3) reinforce behavior. Scientific advances have now identified the ability of tobacco products to (4) trigger strong urges or craving as another important indicator of addictive potential. Here, we propose that these four criteria provide scientifically valid benchmarks that can be used to evaluate the addictiveness of HPFs. Then, we review the evidence regarding whether HPFs meet each criterion. Finally, we consider the implications of labeling HPFs as addictive.

CONCLUSION

Highly processed foods (HPFs) can meet the criteria to be labeled as addictive substances using the standards set for tobacco products. The addictive potential of HPFs may be a key factor contributing to the high public health costs associated with a food environment dominated by cheap, accessible and heavily marketed HPFs.

Rats learn to prefer the late-consumed flavor over the early-consumed flavor in a multi-flavored meal paired with oral glucose and corn oil

Conditioned flavor preference (CFP) is established by association: where a neutral flavor (conditioned stimulus, CS) is paired with orosensory and post-ingestive components of nutrients, including sugar and fat (unconditioned stimulus, US). A previous study reported that rats can learn to prefer flavors that they consumed earlier and later in a multi-flavored solution paired with an intragastric infusion of glucose, but they expressed only a preference for a late-consumed flavor when they were tested after feeding (Myers and Whitney, 2011). This paradigm can be a suitable rodent model to explain how humans acquire a selective preference for routinely late-served "dessert" foods and why these foods remain attractive even in the absence of hunger. Here, we examined whether oral glucose (Experiment 1) or fat (Experiment 2) acts as a US for flavor preference learning processes in this paradigm. In Experiment 1, adult female rats under food restriction were trained in 16 daily sessions with two distinct flavor CSs in succession per session; eight CS(+) sessions in which two distinct flavor CSs (early(+), late(+)) were sequentially presented for 8 min each with oral glucose (12%) as a US, and eight CS(-) sessions in which different CSs (early(-), late(-)) were unpaired with the US. In the 30-minute two-bottle choice test, rats preferred late(+) over late(-) only when tested 90 min after consumption of normal chow (fed test) but not after overnight deprivation (hungry test). Early(+) was not preferred over early(-) in both tests. Moreover, a significant preference for late(+) over early(+) was observed only in the fed test, which is a unique feature of oral glucose-CFP. These results indicate that taste sensations of oral glucose promote a rewarding effect of late-onset glucose nutrients. In Experiment 2, separate rats were trained with the same conditioning paradigm, but used a caloric matched fat solution (5.3% corn oil) for a US. The results showed that they expressed stronger preferences for early(+) and late(+) relative to their respective CS(-) flavors in both tests. Similar to Experiment 1, it was observed in the fed test that there was a preference for late(+) over early(+) in oral fat-CFP. Taken together, the present results suggest that routine timing arrangements can cause qualitative differences in conditioned preferences between multiple flavors within a sugar or fat-containing meal in rats, and that rats prefer the late-consumed flavor over the early-consumed flavor in the absence of hunger.

Learning of food preferences: mechanisms and implications for obesity & metabolic diseases

Omnivores, including rodents and humans, compose their diets from a wide variety of potential foods. Beyond the guidance of a few basic orosensory biases such as attraction to sweet and avoidance of bitter, they have limited innate dietary knowledge and must learn to prefer foods based on their flavors and postoral effects. This review focuses on postoral nutrient sensing and signaling as an essential part of the reward system that shapes preferences for the associated flavors of foods. We discuss the extensive array of sensors in the gastrointestinal system and the vagal pathways conveying information about ingested nutrients to the brain. Earlier studies of vagal contributions were limited by nonselective methods that could not easily distinguish the contributions of subsets of vagal afferents. Recent advances in technique have generated substantial new details on sugar- and fat-responsive signaling pathways. We explain methods for conditioning flavor preferences and their use in evaluating gut-brain communication. The SGLT1 intestinal sugar sensor is important in sugar conditioning; the critical sensors for fat are less certain, though GPR40 and 120 fatty acid sensors have been implicated. Ongoing work points to particular vagal pathways to brain reward areas. An implication for obesity treatment is that bariatric surgery may alter vagal function.

Capsaicin-induced visceral deafferentation does not attenuate flavor conditioning by intragastric fat infusions in mice

The postoral actions of sugar and fat can rapidly stimulate the intake of and preference for flavors associated with these nutrients via a process known as appetition. Prior findings revealed that postoral glucose appetition is not attenuated following capsaicin-induced visceral deafferentation. The present experiment determined if capsaicin treatment altered fat appetition in C57BL/6 mice. Following capsaicin (Cap) or control (Con) treatment, mice were fitted with chronic intragastric (IG) catheters. They were then given 1-h sessions with a flavored saccharin solution (CS-) paired with IG water infusion or a different flavor (CS+) paired with IG 6.4% fat infusion. IG fat stimulated CS+ intakes in both Cap and Con mice, and the groups displayed similar preferences for CS+ over CS- in two-choice tests. These results confirm prior reports of normal fat conditioning in rats exposed to capsaicin or vagal deafferentation surgery. In contrast, other recent findings indicate that total or selective vagotomy alters the preference of mice for dilute vs. concentrated fat sources.

Fat, carbohydrate and protein by oral gavage in the rat can be equally effective for satiation

This study aimed to determine the relative efficacy of the macronutrients, protein, fat and carbohydrate to induce satiation and satiety in rats in relation to macronutrient activation of neurons in the nucleus of the solitary tract (NTS). Male Sprague Dawley rats were schedule-fed twice a day for 2 h, receiving 100% of daily ad-libitum energy intake. On test day 1, 30 min before the first scheduled meal of the day, rats were gavaged with an 8 kcal isocaloric, isovolumetric solution of a glucose, lipid or peptone macronutrient solution or a non-caloric saline solution. To assess satiation, thirty minutes later rats were given access to food for 2 h and food intake determined. A second 2 h food access period 3 h later was used for assessment of satiety. On the second test day, rats were gavaged as before and killed 90 min after food presentation. Blood was collected for measurement of circulating metabolic markers. Brains were removed for analysis of c-Fos expression by in situ hybridization in the NTS. Rats which received saline consumed a similar amount of food compared to pre-gavage intakes. However, rats gavaged with a caloric macronutrient solution all reduced food intake by 18-20 kcal. Interestingly, the reduction in caloric intake was greater than the caloric value of the macronutrient solution gavaged and was sustained following the second scheduled meal. Quantification by in situ hybridization of c-Fos mRNA expression in the NTS 90 min post-gavage, showed a significant increase with each macronutrient, but was 24-29% higher with a lipid or peptone gavage compared to a glucose gavage. In conclusion, when delivered directly to the stomach, all macronutrients can be equally effective in inducing satiation with significant neuronal activation in the NTS of the hindbrain.

Optimizing reproducibility of operant testing through reinforcer standardization: identification of key nutritional constituents determining reward strength in touchscreens

Reliable and reproducible assessment of animal learning and behavior is a central aim of basic and translational neuroscience research. Recent developments in automated operant chamber technology have led to the possibility of universal standard protocols, in addition to increased translational potential, reliability and accuracy. However, the impact of regional and national differences in the supplies of available reinforcers in this system on behavioural performance and inter-laboratory variability is an unknown and at present uncontrolled variable. Therefore, we aimed to identify which constituent(s) of the reward determines reinforcer strength to enable improved standardization of this parameter across laboratories. Male C57BL/6 mice were examined in the touchscreen-based fixed ratio (FR) and progressive ratio (PR) schedules, reinforced with different kinds of milk-based reinforcers to directly compare the incentive values of plain milk (PM, high-calorie: high-fat/low-sugar), strawberry-flavored milk (SM, high-calorie: low-fat/high-sugar), and semi-skimmed low-fat milk (LM, low-calorie: low-fat/low-sugar) on the basis of differences in caloric content, sugar/fat content, and flavor. Use of a higher caloric content reward was effective in increasing operant training acquisition rate. Total trial number completed in FR and breakpoint in PR were higher using the two isocaloric milk products (PM and SM) than the lower caloric LM, with comparable outcomes between PM and SM conditions, suggesting that total caloric content determines reward strength. Analysis of within-session changes in response rate revealed that overall outputs in FR and PR primarily depend on the response rate at the initial phase of a session, which itself was dependent on reinforcer caloric content. Interestingly, the rate of satiation, indicated by decay in response rate within a FR session, was highest when reinforced with SM, suggesting a rapid satiating effect of sugar. The key contribution of reward caloric content to operant performance was confirmed in a multi-laboratory study using the touchscreen 5-choice serial reaction time task (5-CSRTT) reinforced by two isocaloric milk-based liquid rewards with different countries of origin, which yielded consistent performance parameters across sites. Our results indicate that milk-based liquid reinforcer standardization can be facilitated by matching caloric content across laboratories despite regional or national differences in other non-caloric aspects of the reinforcers.

Anthony Sclafani: Consummate scientist

In this article we review the scientific contributions of Anthony Sclafani, with specific emphasis on his early work on the neural substrate of the ventromedial hypothalamic (VMH) hyperphagia-obesity syndrome, and on the development of diet-induced obesity (DIO). Over a period of 20 years Sclafani systematically investigated the neuroanatomical basis of the VMH hyperphagia-obesity syndrome, and ultimately identified a longitudinal oxytocin-containing neural tract contributing to its expression. This tract has since been implicated in mediating the effects of at least two gastrointestinal satiety factors. Sclafani was one of the first investigators to demonstrate DIO in rats as a result of exposure to multiple palatable food items (the "supermarket diet"), and concluded that diet palatability was the primary factor responsible for DIO. Sclafani went on to investigate the potency of specific carbohydrate and fat stimuli for inducing hyperphagia, and in so doing discovered that post-ingestive nutrient effects contribute to the elevated intake of palatable food items. To further investigate this effect, he devised an intragastric infusion system which allowed the introduction of nutrients into the gut paired with the oral intake of flavored solutions, an apparatus her termed the "electronic esophagus". Sclafani coined the term "appetition" to describe the effect of intestinal nutrient sensing on post-ingestive appetite stimulation. Sclafani's productivity in the research areas he chose to investigate has been nothing short of extraordinary, and his studies are characterized by inventive hypothesizing and meticulous experimental design. His results and conclusions, to our knowledge, have never been contradicted.

Western diet and the weakening of the interoceptive stimulus control of appetitive behavior

In obesogenic environments food-related external cues are thought to overwhelm internal cues that normally regulate energy intake. We investigated how this shift from external to internal stimulus control might occur. Experiment 1 showed that rats could use stimuli arising from 0 and 4h food deprivation to predict sucrose delivery. Experiment 2 then examined (a) the ability of these deprivation cues to compete with external cues and (b) how consuming a Western-style diet (WD) affects that competition. Rats were trained to use both their deprivation cues and external cues as compound discriminative stimuli. Half of the rats were then placed on WD while the others remained on chow, and external cues were removed to assess learning about deprivation state cues. When tested with external cues removed, chow-fed rats continued to discriminate using only deprivation cues, while WD-fed rats did not. The WD-fed group performed similarly to control groups trained with a noncontingent relationship between deprivation cues and sucrose reinforcement. Previous studies provided evidence that discrimination based on interoceptive deprivation cues depends on the hippocampus and that WD intake could interfere with hippocampal functioning. A third experiment assessed the effects of neurotoxic hippocampal lesions on weight gain and on sensitivity to the appetite-suppressing effects of the satiety hormone cholecystokinin (CCK). Relative to controls, hippocampal-lesioned rats gained more weight and showed reduced sensitivity to a 1.0ug but not 2.0 or 4.0ug CCK doses. These findings suggest that WD intake reduces utilization of interoceptive energy state signals to regulate appetitive behavior via a mechanism that involves the hippocampus.

The role of dopamine in the pursuit of nutritional value

Acquiring enough food to meet energy expenditure is fundamental for all organisms. Thus, mechanisms have evolved to allow foods with high nutritional value to be readily detected, consumed, and remembered. Although taste is often involved in these processes, there is a wealth of evidence supporting the existence of taste-independent nutrient sensing. In particular, post-ingestive mechanisms arising from the arrival of nutrients in the gut are able to drive food intake and behavioural conditioning. The physiological mechanisms underlying these effects are complex but are believed to converge on mesolimbic dopamine signalling to translate post-ingestive sensing of nutrients into reward and reinforcement value. Discerning the role of nutrition is often difficult because food stimulates sensory systems and post-ingestive pathways in concert. In this mini-review, I discuss the various methods that may be used to study post-ingestive processes in isolation including sham-feeding, non-nutritive sweeteners, post-ingestive infusions, and pharmacological and genetic methods. Using this structure, I present the evidence that dopamine is sensitive to nutritional value of certain foods and examine how this affects learning about food, the role of taste, and the implications for human obesity.

Intragastric fat self-administration is impaired in GPR40/120 double knockout mice

Mice acquire strong preferences for flavors paired with intragastric (IG) fat infusions. This IG fat conditioning is attenuated in double knockout (DoKO) mice missing GPR40 and GPR120 fatty acid receptors. Here we determined if GPR40/120 DoKO mice are also impaired in IG fat self-administration in an operant lick task. In daily 1-h sessions the mice were trained with a sipper spout that contained dry food pellets; licks on the spout triggered infusions of IG fat (Intralipid). The training sessions were followed by test sessions with an empty spout. GPR40/120 DoKO mice self-infused more 20% fat than wild type (WT) C57BL/6 mice in training with a food-baited spout (2.4 vs. 2.0kcal/h) but self-infused less 20% fat than WT mice in empty spout tests (1.2 vs. 1.7kcal/h). The DoKO mice also self-infused less 5% fat than WT mice (0.6 vs. 1.3kcal/h) although both groups emitted more licks for 5% fat than 20% fat. The DoKO and WT mice did not differ, however, in their self-infusion of 12.5% glucose (1.5 vs. 1.6kcal/h), which is isocaloric to 5% fat. A second 5% IL test showed that the DoKO mice reverted to a reduced self-infusion compared to WT mice. When the infusion was shifted to water, WT mice reduced licking in the first extinction session, whereas DoKO mice were less sensitive to the absence of infused fat. Our results indicate that post-oral GPR40/120 signaling is not required to process IG fat infusions in food-baited spout training sessions but contributes to post-oral fat reinforcement in empty spout tests and flavor conditioning tests.

Rats eat a cafeteria-style diet to excess but eat smaller amounts and less frequently when tested with chow

Background

Obesity is associated with excessive consumption of palatable, energy dense foods. The present study used an animal model to examine feeding patterns during exposure to and withdrawal from these foods.

Methods

Male Sprague Dawley rats were exposed to standard lab chow only (Chow rats) or a range of cafeteria-style foods eaten by people (Caf rats). After 1, 4, 7 and 10 weeks of diet in their home cage, rats were subjected to 24-hour test sessions in a Comprehensive Lab Animal Monitoring System (CLAMS). In the first two test sessions, Chow rats were exposed to standard lab chow only while Caf rats were exposed to a biscuit and high-fat chow diet. In the final two test sessions, half the rats in each group were switched to the opposing diet. In each session we recorded numbers of bouts, energy consumed per bout, and intervals between bouts across the entire 24 hours.

Results

Relative to Chow rats, Caf rats initiated fewer bouts but consumed more energy per bout; however, their motivation to feed in the CLAMS declined over time, which was attributed to reduced variety of foods relative to their home cage diet. This decline in motivation was especially pronounced among Caf rats switched from the palatable CLAMS diet to standard lab chow only: the reduced energy intake in this group was due to a modest decline in bout frequency and a dramatic decline in bout size.

Conclusions

Exposure to a cafeteria-diet, rich in variety, altered feeding patterns, reduced rats' motivation to consume palatable foods in the absence of variety, and further diminished motivation to feed when palatable foods were withdrawn and replaced with chow. Hence, variety is a key factor in driving excessive consumption of energy dense foods, and therefore, excessive weight gain.

Post-oral fat stimulation of intake and conditioned flavor preference in C57BL/6J mice: A concentration-response study

Fat appetite is determined not only by orosensory (flavor) stimuli but also by the post-oral actions of dietary fat, which promote increased attraction to the flavors of high-fat foods. Experiment 1 presents a concentration-response analysis of how intragastric (IG) fat self-infusions stimulate intake and condition flavor preferences in C57BL/6J mice trained 1h/day. Separate groups of food-restricted mice consumed a flavored saccharin solution (the CS-) paired with IG self-infusions of water (Test 0) followed by a different flavored solution (the CS+) paired with IG self-infusions of 1.6, 3.2, 6.4 or 12.8% Intralipid (IL, soybean oil) (Tests 1-3). Following additional CS- and CS+ training sessions, a two-bottle CS+ vs. CS- choice test was conducted without infusions. Infusions of 3.2-12.8% IL stimulated CS+ licking in the first test session and more so in subsequent test sessions, and also conditioned significant CS+ preferences. These effects were similar to those previously observed with isocaloric glucose infusions (8-32%). IG infusion of 1.6% IL stimulated intake slightly but did not condition a CS+ preference comparable to the actions of isocaloric 4% glucose. Experiment 2 compared these subthreshold IL and glucose concentrations with that of a 1.6% IL+4% glucose infusion. This mixture stimulated 1-h CS+ licking more rapidly but generated a preference similar to that of 1.6% IL. In 23h/day tests, however, the IL+glucose mixture stimulated greater CS+ intakes and preferences than did 1.6% IL or 4% glucose. These findings show that fat, like glucose, rapidly generates concentration-dependent post-oral signals that stimulate intake and enhance preferences for energy-rich foods in mice.

Rats acquire stronger preference for flavors consumed towards the end of a high-fat meal

Rats learn to prefer flavors associated with postingestive effects of nutrients. The physiological signals underlying this postingestive reward are unknown. We have previously shown that rats readily learn to prefer a flavor that was consumed early in a multi-flavored meal when glucose is infused intragastrically (IG), suggesting rapid postingestive reward onset. The present experiments investigate the timing of postingestive fat reward, by providing distinctive flavors in the first and second halves of meals accompanied by IG fat infusion. Learning stronger preference for the earlier or later flavor would indicate when the rewarding postingestive effects are sensed. Rats consumed sweetened, calorically-dilute flavored solutions accompanied by IG high-fat infusion (+ sessions) or water (- sessions). Each session included an "Early" flavor for 8min followed by a "Late" flavor for 8min. Learned preferences were then assessed in two-bottle tests (no IG infusion) between Early(+) vs. Early(-), Late(+) vs. Late(-), Early(+) vs. Late(+), and Early(-) vs. Late(-). Rats only preferred Late(+), not Early(+), relative to their respective (-) flavors. In a second experiment rats trained with a higher fat concentration learned to prefer Early(+) but more strongly preferred Late(+). Learned preferences were evident when rats were tested deprived or recently satiated. Unlike with glucose, ingested fat appears to produce a slower-onset rewarding signal, detected later in a meal or after its termination, becoming more strongly associated with flavors towards the end of the meal. This potentially contributes to enhanced liking for dessert foods, which persists even when satiated.

Eating Motives and the Controversy over Dieting: Eating Less Than Needed versus Less Than Wanted

Anti-dieting sentiment has grown in recent years. Critics of restrained eating suggest that it evokes counter-regulatory responses that render it ineffective or even iatrogenic. However, restrained eaters are not in negative energy balance and overweight individuals show reduced eating problems when losing weight by dieting. A distinction is often drawn between physiological and psychological hunger, and neuroscience research has shown that there is a neurophysiological reality underlying this distinction. The brain has a homeostatic system (activated by energy deficits) and a hedonic system (activated by the presence of palatable food). The omnipresence of highly palatable food in the environment may chronically activate the hedonic appetite system, producing a need to actively restrain eating not just to lose weight but to avoid gaining it. Just as restricting energy intake below homeostatic needs produces physiological deprivation, restricting intake of palatable foods may produce "perceived deprivation" despite a state of energy balance. In summary, the motivation to eat more than one needs appears to be every bit as real, and perhaps every bit as powerful, as the motivation to eat when energy deprived.

Effects of CB1 and CRF1 receptor antagonists on binge-like eating in rats with limited access to a sweet fat diet: lack of withdrawal-like responses

Positive reinforcement (e.g., appetitive, rewarding properties) has often been hypothesized to maintain excessive intake of palatable foods. Recently, rats receiving intermittent access to high sucrose diets showed binge-like intake with withdrawal-like signs upon cessation of access, suggesting negative reinforcement mechanisms contribute as well. Whether intermittent access to high fat diets also produces withdrawal-like syndromes is controversial. The present study therefore tested the hypothesis that binge-like eating and withdrawal-like anxiety would arise in a novel model of binge eating based on daily 10-min access to a sweet fat diet (35% fat kcal, 31% sucrose kcal). Within 2-3 weeks, female Wistar rats developed binge-like intake comparable to levels seen previously for high sucrose diets (~40% of daily caloric intake within 10 min) plus excess weight gain and adiposity, but absent increased anxiety-like behavior during elevated plus-maze or defensive withdrawal tests after diet withdrawal. Binge-like intake was unaffected by pretreatment with the corticotropin-releasing factor type 1 (CRF(1)) receptor antagonist R121919, and corticosterone responses to restraint stress did not differ between sweet-fat binge rats and chow-fed controls. In contrast, pretreatment with the cannabinoid type 1 (CB(1)) receptor antagonist SR147778 dose-dependently reduced binge-like intake, albeit less effectively than in ad lib chow or sweet fat controls. A priming dose of the sweet fat diet did not precipitate increased anxiety-like behavior, but rather increased plus-maze locomotor activity. The results suggest that CB(1)-dependent positive reinforcement rather than CRF(1)-dependent negative reinforcement mechanisms predominantly maintain excessive intake in this limited access model of sweet-fat diet binges.

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.

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.

Intragastric infusion of glucose enhances the rewarding effect of sorbitol fatty acid ester ingestion as measured by conditioned place preference in mice

We investigated substances that induce a rewarding effect during the postingestive process using the conditioned place preference (CPP) test. Although mice showed high affinity for a low-energy fat substitute--sorbitol fatty acid esters and low-concentration linoleic acid solution--they did not exhibit a place preference toward a voluntary intake of fat substitute in the CPP test. However, during a conditioning session of CPP that involved intragastric administration of corn oil immediately before the intake of the fat substitute, the test mice displayed a place preference. Similarly, intragastric administration of glucose, galactose, and dextrin also induced CPP; however, fructose, mannose, and a nonmetabolized carbohydrate did not. These results suggest that administration of corn oil and glucose has the same postingestive effect with regard to inducing CPP and that the structural specificity of carbohydrates influences the postingestive effect.

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.

Circulating triglycerides after a high-fat meal: predictor of increased caloric intake, orexigenic peptide expression, and dietary obesity

Recent studies in normal-weight rats have linked circulating triglycerides (TG), when elevated by a high-fat (HF) compared to equicaloric low-fat (LF) meal, to an increase in subsequent food intake and hypothalamic expression of orexigenic peptides. The present study tested whether natural variations between rats in their TG levels after a small HF meal can also be related to their individual patterns of eating and peptide expression. In tail vein blood collected on three separate days 60 min after a HF meal, levels of TG were found to be strongly, positively correlated within rats from day to day but were highly variable between rats (75-365 mg/dl), allowing distinct subgroups (33% lowest or highest) to be formed. Compared to "Low-TG responders" with post-meal levels averaging 109 mg/dl, "High-TG responders" with 240 mg/dl showed in two separate experiments a significant increase in caloric intake in a subsequent laboratory chow meal. Before this larger meal, these rats with elevated TG consistently exhibited higher expression levels and synthesis of the orexigenic peptides, enkephalin, orexin and melanin-concentrating hormone, as revealed using real-time quantitative PCR, radiolabeled in situ hybridization, and immunofluorescence histochemistry. Over the long-term, the High-TG responders also showed an increased propensity to overeat, gain weight and accumulate excess body fat on a chronic HF diet. This simple measure of TG levels after a HF meal may offer a useful tool for identifying subpopulations with increased risk for overeating and dietary obesity and detecting early signs of brain disturbances that may contribute to this high-risk phenotype.

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.

Flavor-nutrient learning is less rapid with fat than with carbohydrate in rats

Flavor-nutrient learning occurs when the post-ingestive consequences of a food are associated with its flavor. As a signal of the food's energy density, flavor-nutrient associations have the potential to contribute to the regulation of meal size. While all calorie sources (fat, carbohydrate, protein, ethanol) can support flavor-nutrient learning, prior research has found that flavor-nutrient associations based on fat may require higher nutrient concentrations and more rigorous experimental protocols than are required to train carbohydrate (cho)-based associations. To further explore potential macronutrient-specific differences in flavor-nutrient learning, the present study compared the time course of acquisition of cho- and fat-based associations. Rats were trained to associate distinctive flavors with high-density (3.2 kcal/mL) and low-density (0.2 kcal/mL) orally-consumed solutions, either fat (corn oil emulsion) or carbohydrate (sucrose). For each nutrient, both within- and between-group designs were used to assess (via two-bottle preference testing) whether flavor-nutrient learning had occurred after 2, 4, or 6 training trial pairs. Rats trained with carbohydrate demonstrated preferential intake of the low-density paired flavor after only 2 training pairs; in contrast, rats trained with fat required 6 training pairs. These findings demonstrate differential rapidity of acquisition flavor-nutrient associations. The longer time course of acquisition of fat-based flavor-nutrient associations may be yet another mechanism by which high-fat foods promote overeating.

Learned flavor preferences. The variable potency of post-oral nutrient reinforcers

The notion that preferences for flavors paired with various nutrients can be attributed simply to their energy content ("flavor-calorie learning") is belied by variation in nutrient reinforcing potency. Fructose, fat and ethanol, all regarded as powerful contributors to food and fluid preferences, are less potent than glucose when their orosensory effects are bypassed. Conditioning studies in animals infused with nutrients as they consume target flavor solutions have shown that the weaker reinforcing effects of these nutrients can be enhanced by various methods that improve the opportunity for associating a flavor with post-oral effects. Until the nature of the reinforcing stimuli is understood, "flavor-nutrient learning" is a better label for these phenomena.

Sucrose taste but not Polycose taste conditions flavor preferences in rats

Rats have an inborn preference for sweet taste and learn to prefer flavors associated with sweetness. They are also strongly attracted to the taste of glucose polymers (e.g., Polycose). This "poly" taste differs in quality from the sweet taste of sugar. To determine if poly taste, like sweet taste, conditions flavor preferences rats were trained with a distinctive flavor (CS+) added to 2% Polycose solution and a different flavor (CS-) added to plain water. In a subsequent two-bottle test the rats did not prefer the CS+ to CS- when both flavors were presented in water. In contrast, other rats significantly preferred a CS+ flavor that had been paired with 2% sucrose. Adding saccharin to a flavored Polycose solution did not improve CS+ flavor learning; rather, Polycose appeared to overshadow saccharin-induced conditioning. Flavor conditioning by a 16% Polycose solution was assessed using a sham-feeding procedure to prevent post-oral reinforcement. Although the rats sham-fed substantial amounts of the CS+ flavored Polycose solution, they failed to prefer the CS+ to the CS- flavor. This contrasts with the preference other rats displayed for a CS+ paired with sham-fed sucrose. Why attractive sweet and poly tastes differ in their ability to condition flavor preferences is not certain, although some findings suggest that they differentially activate dopamine and/or serotonin circuits involved in flavor learning.

Adaptation of lipid-induced satiation is not dependent on caloric density in rats

Food intake is modulated by ingestive (gastrointestinal) and post-ingestive signals; ingested fat is potent to produce short-term satiety (satiation) but this can be modified by long-term ingestion of a high fat diet.

AIM

Determine whether altered lipid-induced satiation is dependent on the fat content of the diet, rather than increased caloric density or changes in adiposity.

METHODS

Initial experiments determined the differences in the microstructure of meal patterns in rats fed a high fat diet (HF: 38% fat kcal) and in rats pair-fed an isocaloric, isonitrogenous low fat diet (LF: 10% fat kcal) and changes in meal patterns measured after long-term maintenance on the HF diet.

RESULTS

Rats fed the HF diet had a significant 50% increase in meal frequency compared to rats fed the LF diet; in addition, there was a significant reduction in meal size (32%) and inter meal interval (38%) consistent with induction of satiation. After 8 weeks on the HF diet, these parameters tend to approach those of rats maintained on the LF diet. There was a significant 56% decrease in the activation of neurons in the NTS in response to intragastric gavage of lipid in rats maintained for 8 weeks on the HF compared to LF diet.

CONCLUSION

Dietary fat alters meal patterns consistent with induction of a short-term satiety signal. This signal is attenuated with long-term exposure to dietary lipid, in the absence of ingestion of additional calories or changes in body weight. This adaptation of short-term satiety might contribute to diet-induced obesity.

Increased caloric intake after a high-fat preload: relation to circulating triglycerides and orexigenic peptides

To investigate mechanisms that mediate the greater food intake induced by a fat-rich diet, the present study tested an acute "preload-to-test meal" paradigm in normal-weight rats. In this paradigm, the rats were given a small high-fat (HF) compared to low-fat (LF) preload and, after an intermeal interval, allowed to consume freely on a subsequent test meal. Modified versions of this paradigm were tested to determine the robustness of the greater caloric intake induced by the HF preload while standardizing the test protocol. A HF preload of 10-15 kcals, compared to an equicaloric LF preload, significantly increased food intake by 40-50% in the subsequent test meal. This effect, a 4-6 kcal increase, was observed with HF preloads equal in energy density and palatability to the LF preloads. It was evident with preloads or test meals that were liquid or solid, preloads that were injected, test meals that had variable fat content, and natural intermeal intervals of 60-120 min. This overeating after a HF preload was invariably associated with a 2- to 3-fold increase in circulating levels of triglycerides (TG), with no change in leptin or insulin. It was also accompanied by increased expression of the orexigenic peptides, galanin in the paraventricular nucleus and orexin in the perifornical lateral hypothalamus. Moreover, if given repeatedly over several days, the HF compared to equicaloric LF preload significantly increased 24-h food intake. These results establish a protocol for studying the phenomenon of increased feeding on a HF diet under controlled conditions and suggest possible underlying mechanisms involving circulating lipids and orexigenic peptides.

Development of learned flavor preferences

Rats, like humans, are born with only a few innate flavor preferences and aversions. Preferences retain great plasticity throughout the lifespan because they are sensitive to modification by experience. From an early age, rats can rapidly learn to prefer or avoid a flavor (conditioned stimulus, CS) that is associated with a positive or negative unconditioned stimulus (US). The US may be the mother's milk, social or thermotactile stimulation, or other food-related stimuli. Flavor-flavor learning occurs when the CS flavor is mixed with a naturally preferred (e.g., sweet) or avoided (e.g., bitter) US flavor. Flavor preferences and aversions are also produced by USs that have postoral positive (e.g., nutritious) or negative (e.g., toxic) actions. These types of learning appear to involve different behavioral and neural mechanisms as indicated by differences in conditioned responses, effective temporal parameters, resistance to extinction, and neurochemical mechanisms. New evidence indicates that flavor-nutrient preference learning can occur before weaning and influence food selection after weaning. Flavor conditioning not only affects food choice, but can also significantly increase food acceptance, that is, total consumption. Thus, from an early age, learning processes shape the feeding behavior of animals. While primarily serving an adaptive function, learning may play a role in biasing individuals towards excessive intake and weight gain.

Energy density and macronutrient composition determine flavor preference conditioned by intragastric infusions of mixed diets

In prior studies rats preferred a flavor (CS+HF) paired with intragastric (IG) infusions of a high-fat diet to a flavor (CS+HC) paired with a high-carbohydrate diet, yet just the opposite preference was observed with pure-nutrient infusions. The present study tested the hypothesis that variations in nutrient density as well as composition influence flavor learning. Animals were trained (22 h/day) with IG infusion of milk-based high-fat and high-carbohydrate liquid diets paired with intakes of flavored, noncaloric CS+ solutions. A third flavor, the CS-, was paired with water infusion. Standard chow was available ad libitum. The rats preferred both CS+ flavors to the CS-, whether the infused diets were dense (HF and HC, 2.1 kcal/ml) or dilute (hf and hc, 0.5 kcal/ml), indicating that all diet infusions were reinforcing. They consumed the CS+hc and CS+hf equally in training, and preferred the CS+hc, showing that at low-energy density carbohydrate was more reinforcing than fat. In contrast, CS+HF intake exceeded that of CS+HC in training, and the rats preferred the CS+HF to the CS+HC. In further tests the rats preferred the CS+HF to the CS+hc, the CS+HF to the CS+hf, and the CS+HC to the CS+hc; i.e., when the diets differed in energy density the flavors associated with the more concentrated infusions were preferred. In the absence of influence by flavor cues from the nutrients themselves, rats' preferences for flavors associated with diets high in fat or carbohydrate are dependent on energy density. The differential satiating effects of fat and carbohydrate may contribute to these density-dependent preferences.

Low- and high-carbohydrate diets: body composition differences in rats

OBJECTIVE

To determine differences in adipose tissue mass, cell size, and lipid metabolism transcripts and differences in composition of body weight loss and energy expenditure (EE) after isoenergetic, energy-restricted intake of low-carbohydrate/high-fat/high-protein (LC) and high-carbohydrate/low-fat/moderate-protein (HC) diets.

RESEARCH METHODS AND PROCEDURES

Ten-week old female Sprague-Dawley rats were fed a high-fat diet ad libitum for 8 weeks to induce weight gain and fat deposition. Weight-matched rats were then assigned to isoenergetic LC (Atkins) and HC (American Dietary Association Exchange) diets for 10 weeks at 65% of ad libitum energy intake.

RESULTS

There was no significant difference in the serum lipid profiles or amount of body weight lost between the HC and LC groups, whereas a higher insulin sensitivity index (p < 0.01) resulted from the HC compared with the LC diet. Compared with the post-restriction LC group, the HC group demonstrated (p < 0.05) higher EE during active hours, lower mRNA levels for the lipogenic genes peroxisome proliferator-activated receptor gamma2, lipoprotein lipase, and, adipocyte fatty acid binding protein, decreased adipocyte cell volume, and decreased fat mass.

DISCUSSION

Results indicated down-regulation of lipogenic genes, decreased fat mass, and, therefore, improved body composition in the post-restriction HC compared with the LC group. The small mean differences between the two diet groups (p = 0.11) in 24-hour EE over the 10 weeks of diet intervention would account for the majority of the lower mean body weights in the post-restriction HC group. These data suggest that macronutrient composition of the diet influences body composition and indicate a distinction between HC and LC diets.

Oligofructose promotes satiety in rats fed a high-fat diet: involvement of glucagon-like Peptide-1

OBJECTIVE

To analyze the putative interest of oligofructose (OFS) in the modulation of food intake after high-fat diet in rats and to question the relevance of the expression and secretion of intestinal peptides in that context.

RESEARCH METHODS AND PROCEDURES

Male Wistar rats were pretreated with standard diet or OFS-enriched (10%) standard diet for 35 days followed by 15 days of high-fat diet enriched or not with OFS (10%) treatment. Body weight, food intake, triglycerides, and plasma ghrelin levels were monitored during the treatment. On day 50, rats were food-deprived 8 hours and anesthetized for blood and intestinal tissue sampling for further proglucagon mRNA, glucagon-like peptide (GLP)-1, and GLP-2 quantification.

RESULTS

The addition of OFS in the diet protects against the promotion of energy intake, body weight gain, fat mass development, and serum triglyceride accumulation induced by a high-fat diet. OFS fermentation leads to an increase in proglucagon mRNA in the cecum and the colon and in GLP-1 and GLP-2 contents in the proximal colon, with consequences on the portal concentration of GLP-1 (increase). A lower ghrelin level is observed only when OFS is added to the standard diet of rats.

DISCUSSION

In rats exposed to high-fat diet, OFS is, thus, able to modulate endogenous production of gut peptides involved in appetite and body weight regulation. Because several approaches are currently used to treat type 2 diabetes and obesity with limited effectiveness, dietary fibers such as OFS, which promote the endogenous production of gut peptides like GLP-1, could be proposed as interesting nutrients to consider in the management of fat intake and associated metabolic disorders.

Adaptation to a high-fat diet leads to hyperphagia and diminished sensitivity to cholecystokinin in rats

Rats fed high-fat (HF) diets exhibit reduced sensitivity to some peptide satiety signals. We hypothesized that reduced sensitivity to satiety signals might contribute to overconsumption of a high-energy food after adaptation to HF diets. To test this, we measured daily, 3-h intake of a high-energy, high-fat (HHF, 22.3 kJ/g) test food in rats fed either low-fat (LF) or HF, isoenergetic (16.2 kJ/g) diets. During testing, half of each group received the HHF test food (LF/HHF; HF/HHF), whereas the other half received their respective maintenance diet (LF/LF; HF/HF). Rats fed a HF diet ate more of the HHF food during the 3-h testing period than LF-fed rats (HF/HHF = 7.7 +/- 0.3 g vs. LF/HHF = 5.5 +/- 0.2 g; P = 0.003). Rats tested on their own maintenance diets had similar intakes (HF/HF = 3.2 +/- 0.2 g vs. LF/LF = 3.7 +/- 0.3 g), which were lower (P < or = 0.008) than intakes of rats tested on HHF. HHF-tested rats did not differ in body weight by the end of wk 2 of testing. In a subsequent short-term choice preference test, rats exhibited an equal relative preference for HHF irrespective of their maintenance diets (HF = 63.1%, LF = 68.1%, P = 0.29). Finally, we examined the effect of intraperitoneal NaCl or cholecystokinin (CCK)-8 (100 and 250 ng/kg) injection on 1-h food intake. Both doses of CCK significantly suppressed food intake in LF-fed rats but not HF-fed rats. These results demonstrate that chronic ingestion of a HF diet leads to short-term overconsumption of a high-energy, high-fat food compared with LF-fed cohorts, which is associated with a decreased sensitivity to CCK.

Sugar and fat conditioned flavor preferences in C57BL/6J and 129 mice: oral and postoral interactions

C57BL/6J (B6) mice consume more sugar and fat solutions than do 129 mice in 24-h preference tests. Previous studies have attributed this observation to strain differences in taste responsiveness to these nutrients. We tested the hypothesis that differences in postingestive responsiveness contribute to the strain differences. In experiment 1, B6 and 129 mice were trained to associate consumption of a flavored solution (CS+) with intragastric (IG) infusions of 16% sucrose and a different flavored solution (CS-) with IG water infusions (22 h/day). They were then retrained with new flavors paired with IG infusions of 5.6% soybean oil and water. Although both strains developed preferences for the nutrient-paired CS+ solutions, the B6 mice displayed significantly stronger preferences. The B6 mice consumed more CS+ during training, which may have contributed to their enhanced preference. In a second experiment, training intakes were equated by giving B6 and 129 mice "isosweet" CS solutions prepared with different amounts of sucrose and saccharin. The B6 and 129 mice consumed more of the sugar- or fat-paired CS+ than the water-paired CS- during training. The two strains also displayed equally strong preferences for the CS+ over CS- in choice tests, indicating that they had similar postingestive responsiveness to the sucrose and soybean oil. We propose that B6 mice consume more sugar and fat than 129 mice because their stronger orosensory response stimulates greater intake, which leads to greater stimulation of postingestive nutrient detectors and further enhancement of consumption.

Flavor preferences conditioned by postingestive effects of nutrients in preweanling rats

The purpose of these experiments was to determine if preweanling rats, like adults, are capable of learning to associate an arbitrary flavor with the postingestive effects of nutrients, and then demonstrate a preference for that flavor after weaning. In Experiment 1, preweanlings were trained daily from postnatal day (P) 16 through P19 with intraoral (IO) infusions of a grape or cherry flavor (CS+) mixed with 20% glucose (US), and the opposite flavor (CS-) mixed with 0.05% saccharin. After weaning, rats were given a 4-h two-bottle choice between the CS+ and CS- flavors both presented in 0.05% saccharin. Rats preferred the flavor previously paired with glucose. In Experiment 2 using similar methods, rats learned to prefer a flavor (CS+G) paired with a glucose US over a flavor (CS+S) paired with a sweeter but less nutritive sucrose US, indicating involvement of postingestive reinforcement. In Experiment 3 preweanling rats with IO and intragastric (IG) catheters were trained with a CS+ flavor paired with IG nutrient infusion, and a CS- flavor paired with no IG infusion. These rats showed no flavor preference 3 days after weaning, but did significantly prefer the CS+ flavor over the CS- flavor 10 days after weaning. Together these experiments demonstrate that neural mechanisms for flavor-nutrient associations are developed before weaning, allowing young rats to learn associations between arbitrary flavors and nutritive consequences. Thus nutrient conditioning may be one way that early experience (such as flavors from the maternal diet transmitted in milk) influences later dietary preferences.

The gastrointestinal tract “tastes” nutrients: evidence from the intestinal taste aversion paradigm

To develop and use a behavioral paradigm for assessments of what nutrient properties are detected by intestinal chemoreceptors, we combined features of the “electronic esophagus” preparation (Elizalde G and Sclafani A. Physiol Behav 47: 63–77, 1990) and the conditioned taste aversion protocol (Garcia J and Koelling RA. Psychon Sci 4: 123–124, 1966). In four experiments, separate groups of food-deprived rats with gastric ( experiments 1–4) or duodenal ( experiment 4) catheters were infused with either carbohydrates (maltodextrin) or fats (corn oil) into their stomachs or small intestines, either while they consumed nonnutritive flavored solutions ( experiments 1 and 2) or in the absence of any intake ( experiments 3 and 4). For some animals, one of the macronutrient infusions was paired with lithium chloride injections shown to support conventional conditioned aversions. After training, in various oral preference test trials, animals were given opportunities to taste and consume the nonnutritive solutions that had served as oropharyngeal conditioned stimuli as well as the nutrients that had been infused intragastrically, with or without poisoning, but never sampled by mouth. As previously established, preferences for the nonnutritive flavors were enhanced by association with intragastric infusions of macronutrients, with carbohydrates producing the greater preference. On first exposure to the two macronutrients for oral consumption, animals reduced their intake of the nutrient that had been previously poisoned when it was infused into the gastrointestinal tract. These results, along with additional controls, suggest that nutrient tastes detected in the intestines can be recognized centrally based on oropharyngeal gustatory stimulation.

The relationship between food reward and satiation revisited

The postingestive satiating action of food is often viewed as producing a positive affective state that rewards eating. However, in an early test of this idea, Van Vort and Smith [Physiol. Behav. 30 (1983) 279] reported that rats did not learn to prefer a food that was "real-fed" and satiating over a food that was "sham-fed" and not satiating. Subsequent investigators obtained similar findings with concentrated nutrient sources. With dilute nutrient sources, however, rats learned to prefer the real-fed to the sham-fed food. These and other findings demonstrate that nutrients have rewarding postingestive effects that enhance food preferences via a conditioning process. These reward effects appear separate from the satiating actions of nutrients, which may actually reduce food reward. Food intake and preference are controlled by a complex interaction of positive and negative signals generated by nutrients in the mouth and at postingestive sites.

Ethanol-conditioned flavor preferences compared with sugar- and fat-conditioned preferences in rats

Rats can learn to prefer flavors paired with ethanol and various nutrients. The present study examined the relative strengths of flavor preferences conditioned by 5% ethanol and isocaloric solutions of 7.18% sucrose, 7.18% fructose, or 3.26% corn oil. In three experiments, nondeprived rats were trained with different flavored solutions (conditioned stimuli, CS) paired with intragastric (IG) infusions: a CS+E flavor paired with ethanol infusion, a second CS+ paired with a nutrient infusion, and a CS- paired with water infusion. In two-bottle tests, rats strongly preferred a sucrose-paired CS+S over the CS- and over the CS+E. The preference for the CS+E over CS- was weaker. These effects occurred when the rats drank substantially more CS+S than CS+E in training and when training intakes were matched. Similar results were obtained when the nutrient infusion was fructose or corn oil, except that preferences for the CS+F or CS+O over the CS+E were less pronounced than with CS+S. Consistent with the IG results, rats trained to drink flavored sucrose and ethanol solutions preferred the CS+S to CS+E in a flavored water test. These results confirm prior reports of ethanol-conditioned preferences but show that ethanol is less effective than other nutrients at isocaloric concentrations. The marked individual differences in ethanol-conditioned preferences may be related to the impact of the sugar or fat infusions on the reward evaluation of the ethanol-paired flavor.