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

Appetitive olfactory learning and memory in the honeybee depend on sugar reward identity

One of the most important tasks of the brain is to learn and remember information associated with food. Studies in mice and Drosophila have shown that sugar rewards must be metabolisable to form lasting memories, but few other animals have been studied. Here, we trained adult, worker honeybees (Apis mellifera) in two olfactory tasks (massed and spaced conditioning) known to affect memory formation to test how the schedule of reinforcement and the nature of a sugar reward affected learning and memory. The antennae and mouthparts of honeybees were most sensitive to sucrose but glucose and fructose were equally phagostimulatory. Whether or not bees could learn the tasks depended on sugar identity and concentration. However, only bees rewarded with glucose or sucrose formed robust long-term memory. This was true for bees trained in both the massed and spaced conditioning tasks. Honeybees fed with glucose or fructose exhibited a surge in haemolymph sugar of greater than 120mM within 30s that remained elevated for as long as 20min after a single feeding event. For bees fed with sucrose, this change in haemolymph glucose and fructose occurred with a 30s delay. Our data showed that olfactory learning in honeybees was affected by sugar identity and concentration, but that olfactory memory was most strongly affected by sugar identity. Taken together, these data suggest that the neural mechanisms involved in memory formation sense rapid changes in haemolymph glucose that occur during and after conditioning.

Mechanisms Involved in Guiding the Preference for Fat Emulsion Differ Depending on the Concentration

High-fat foods tend to be palatable and can cause addiction in mice via a reinforcing effect. However, mice showed preference for low fat concentrations that do not elicit a reinforcing effect in a two-bottle choice test with water as the alternative. This behavior indicates the possibility that the mechanism underlying fat palatability may differ depending on the dietary fat content. To address this issue, we examined the influences of the opioid system and olfactory and gustatory transductions on the intake and reinforcing effects of various concentrations of a dietary fat emulsion (Intralipid). We found that the intake and reinforcing effects of fat emulsion were reduced by the administration of an opioid receptor antagonist (naltrexone). Furthermore, the action of naltrexone was only observed at higher concentrations of fat emulsion. The intake and the reinforcing effects of fat emulsion were also reduced by olfactory and glossopharyngeal nerve transections (designated ONX and GLX, respectively). In contrast to naltrexone, the effects of ONX and GLX were mainly observed at lower concentrations of fat emulsion. These results imply that the opioid system seems to have a greater role in determining the palatability of high-fat foods unlike the contribution of olfactory and glossopharyngeal nerves.

Operant licking for intragastric sugar infusions: Differential reinforcing actions of glucose, sucrose and fructose in mice

Intragastric (IG) flavor conditioning studies in rodents indicate that isocaloric sugar infusions differ in their reinforcing actions, with glucose and sucrose more potent than fructose. Here we determined if the sugars also differ in their ability to maintain operant self-administration by licking an empty spout for IG infusions. Food-restricted C57BL/6J mice were trained 1 h/day to lick a food-baited spout, which triggered IG infusions of 16% sucrose. In testing, the mice licked an empty spout, which triggered IG infusions of different sugars. Mice shifted from sucrose to 16% glucose increased dry licking, whereas mice shifted to 16% fructose rapidly reduced licking to low levels. Other mice shifted from sucrose to IG water reduced licking more slowly but reached the same low levels. Thus IG fructose, like water, is not reinforcing to hungry mice. The more rapid decline in licking induced by fructose may be due to the sugar's satiating effects. Further tests revealed that the Glucose mice increased their dry licking when shifted from 16% to 8% glucose, and reduced their dry licking when shifted to 32% glucose. This may reflect caloric regulation and/or differences in satiation. The Glucose mice did not maintain caloric intake when tested with different sugars. They self-infused less sugar when shifted from 16% glucose to 16% sucrose, and even more so when shifted to 16% fructose. Reduced sucrose self-administration may occur because the fructose component of the disaccharide reduces its reinforcing potency. FVB mice also reduced operant licking when tested with 16% fructose, yet learned to prefer a flavor paired with IG fructose. These data indicate that sugars differ substantially in their ability to support IG self-administration and flavor preference learning. The same post-oral reinforcement process appears to mediate operant licking and flavor learning, although flavor learning provides a more sensitive measure of sugar reinforcement.

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.

Flavor preference conditioning by different sugars in sweet ageusic Trpm5 knockout mice

Knockout (KO) mice missing the taste signaling protein Trpm5 have greatly attenuated sweetener preferences but develop strong preferences for glucose in 24-h tests, which is attributed to post-oral sugar conditioning. Trpm5 KO mice express mild preferences for galactose but no preferences for fructose in 24-h tests, which suggests that these sugars differ in their post-oral reinforcing effects. Here we investigated sugar-conditioned flavor preferences in Trpm5 KO and C57BL/6J wildtype (B6) mice. The mice were trained to consume a flavored (CS+, e.g. grape) 8% sugar solution and flavored (CS-, e.g., cherry) water on alternating days followed by two-bottle choice tests with CS+ vs. CS- flavors in water and with unflavored sugar vs. water. The KO mice displayed strong preferences (>80%) for the CS+ glucose and CS+ galactose but not for the CS+ fructose flavor. They also preferred glucose and galactose, but not fructose to water. In contrast, the B6 mice preferred all three CS+ flavors to the CS- flavor, and all three sugars to water. In tests with the non-metabolizable sugar α-methyl-d-glucopyranoside (MDG), the KO and B6 mice preferred 8% MDG to water but did not prefer the CS+ 8% MDG to CS-. However, they preferred a CS+ flavor mixed with 4% MDG over the CS- flavor. Trpm5 KO mice also preferred galactose and MDG to fructose in direct choice tests. The Trpm5 KO data indicate that glucose and, to a lesser extent, galactose and MDG have post-oral reinforcing actions that stimulate intake and preference while fructose has a much weaker effect. The CS+ flavor and sugar preferences of B6 mice may be mediated by the sweet taste and/or post-oral actions of the various sugars. Glucose, galactose, and MDG, but not fructose, are ligands for the sodium-glucose transporter 1 (SGLT1) which is implicated in post-oral sugar conditioning in B6 mice.

Classical conditioning and pain: conditioned analgesia and hyperalgesia

This article reviews situations in which stimuli produce an increase or a decrease in nociceptive responses through basic associative processes and provides an associative account of such changes. Specifically, the literature suggests that cues associated with stress can produce conditioned analgesia or conditioned hyperalgesia, depending on the properties of the conditioned stimulus (e.g., contextual cues and audiovisual cues vs. gustatory and olfactory cues, respectively) and the proprieties of the unconditioned stimulus (e.g., appetitive, aversive, or analgesic, respectively). When such cues are associated with reducers of exogenous pain (e.g., opiates), they typically increase sensitivity to pain. Overall, the evidence concerning conditioned stress-induced analgesia, conditioned hyperalagesia, conditioned tolerance to morphine, and conditioned reduction of morphine analgesia suggests that selective associations between stimuli underlie changes in pain sensitivity.

Post-oral appetite stimulation by sugars and nonmetabolizable sugar analogs

Post-oral sugar actions enhance the intake of and preference for sugar-rich foods, a process referred to as appetition. Here, we investigated the role of intestinal sodium glucose cotransporters (SGLTs) in sugar appetition in C57BL/6J mice using sugars and nonmetabolizable sugar analogs that differ in their affinity for SGLT1 and SGLT3. In experiments 1 and 2, food-restricted mice were trained (1 h/day) to consume a flavored saccharin solution [conditioned stimulus (CS-)] paired with intragastric (IG) self-infusions of water and a different flavored solution (CS+) paired with infusions of 8 or 12% sugars (glucose, fructose, and galactose) or sugar analogs (α-methyl-D-glucopyranoside, MDG; 3-O-methyl-D-glucopyranoside, OMG). Subsequent two-bottle CS+ vs. CS- choice tests were conducted without coinfusions. Infusions of the SGLT1 ligands glucose, galactose, MDG, and OMG stimulated CS+ licking above CS- levels. However, only glucose, MDG, and galactose conditioned significant CS+ preferences, with the SGLT3 ligands (glucose, MDG) producing the strongest preferences. Fructose, which is not a ligand for SGLTs, failed to stimulate CS+ intake or preference. Experiment 3 revealed that IG infusion of MDG+phloridzin (an SGLT1/3 antagonist) blocked MDG appetition, whereas phloridzin had minimal effects on glucose-induced appetition. However, adding phloretin (a GLUT2 antagonist) to the glucose+phloridzin infusion blocked glucose appetition. Taken together, these findings suggest that humoral signals generated by intestinal SGLT1 and SGLT3, and to a lesser degree, GLUT2, mediate post-oral sugar appetition in mice. The MDG results indicate that sugar metabolism is not essential for the post-oral intake-stimulating and preference-conditioning actions of sugars in mice.

Impact of T1r3 and Trpm5 on carbohydrate preference and acceptance in C57BL/6 mice

Knockout (KO) mice missing the sweet taste receptor subunit T1r3 or the signaling protein Trpm5 have greatly attenuated sweetener preferences but learn to prefer sucrose in 24-h tests. Here, we examined 24-h preferences of T1r3 KO, Trpm5 KO, and C57BL/6J wild-type (WT) mice for glucose, fructose, galactose, and corn starch. Unlike glucose, fructose has little postoral reward effect in WT mice, whereas conflicting data have been obtained with galactose. Naïve KO mice were initially indifferent to dilute glucose solutions (0.5-4%) but exhibited strong preferences for 8-32% concentrations. In a second test, they strongly preferred (~90%) all glucose concentrations although they drank less sugar than WT mice. Naïve KO mice were indifferent to 0.5-8% fructose and avoided 16-32% fructose. However, the glucose-experienced KO mice displayed significant preferences for all fructose solutions. Naïve KO mice preferred only 8% galactose, whereas WT mice preferred 4-16% galactose, and all mice avoided 32% galactose. Galactose experience enhanced the preference for this sugar in KO and WT mice. Naïve T1r3 KO and WT mice displayed similar preferences for 0.5-32% corn starch, which were enhanced by starch experience. Naïve Trpm5 KO mice did not prefer starch but did so after 1-bottle starch experience. The results confirm the sweet taste deficits of T1r3 KO and Trpm5 KO mice but demonstrate their ability to develop strong glucose and milder galactose preferences attributed to the postoral actions of these sugars. The acquired preference for the non-sweet flavor properties of glucose generalized to those of fructose. The findings further demonstrate that although Trpm5 (but not T1r3) signaling is essential for starch preference, Trpm5 KO mice can learn to prefer starch based on its postoral effects.

Flavor preferences conditioned by intragastric glucose but not fructose or galactose in C57BL/6J mice

The present study determined if mice, like rats, differ in their flavor conditioning responses to intragastric (IG) infusions of three common monosaccharide sugars. In Experiment 1, C57BL/6J mice were trained to drink a flavored saccharin solution (the CS+) paired with intragastric (IG) self-infusions of 16% glucose, fructose or galactose and a different flavored solution (the CS-) paired with IG water infusions during 22 h/day training sessions. The glucose infusions increased CS+ intakes during training and produced a strong CS+ preference (~87%) in two-bottle choice tests. In contrast, the fructose and galactose infusions reduced CS training intakes and did not condition a CS+ preference. Experiment 2 determined if reducing fructose and galactose concentration would enhance conditioning. However, IG infusions of 8% sugar also failed to condition CS+ preferences. The robust conditioning response to IG glucose confirms results obtained with rats, but the indifference of mice to IG fructose and galactose contrasts with preference and avoidance responses observed in rats. The effectiveness of glucose to condition preferences suggests an important role for glucose-specific sensors rather than gut "sweet" taste receptors in the postoral modulation of carbohydrate appetite.

High fat diet intake during pre and periadolescence impairs learning of a conditioned place preference in adulthood

BACKGROUND

Brain regions that mediate learning of a conditioned place preference (CPP) undergo significant development in pre and periadolescence. Consuming a high fat (HF) diet during this developmental period and into adulthood can lead to learning impairments in rodents. The present study tested whether HF diet intake, consumed only in pre and periadolescence, would be sufficient to cause impairments using a CPP procedure.

METHODS

Rats were randomly assigned to consume a HF or a low fat (LF) diet during postnatal days (PD) 21-40 and were then placed back on a standard lab chow diet. A 20-day CPP procedure, using HF Cheetos® as the unconditioned stimulus (US), began either the next day (PD 41) or 40 days later (PD 81). A separate group of adult rats were given the HF diet for 20 days beginning on PD 61, and then immediately underwent the 20-day CPP procedure beginning on PD 81.

RESULTS

Pre and periadolescent exposure to a LF diet or adult exposure to a HF diet did not interfere with the development of a HF food-induced CPP, as these groups exhibited robust preferences for the HF Cheetos® food-paired compartment. However, pre and periadolescent exposure to the HF diet impaired the development of a HF food-induced CPP regardless of whether it was assessed immediately or 40 days after the exposure to the HF diet, and despite showing increased consumption of the HF Cheetos® in conditioning.

CONCLUSIONS

Intake of a HF diet, consumed only in pre and periadolescence, has long-lasting effects on learning that persist into adulthood.