Glucose-conditioned flavor preference learning requires co-activation of NMDA and dopamine D1-like receptors within the amygdala

Reconsolidation of a post-ingestive nutrient memory requires mTOR in the central amygdala

The central control of feeding behavior and metabolic homeostasis has been proposed to involve a form of post-ingestive nutrient learning independent of the gustatory value of food. However, after such learning, it is unknown which brain regions or circuits are activated to retrieve the stored memory and whether this memory undergoes reconsolidation that depends on protein synthesis after its reactivation through retrieval. In the present study, using a conditioned-flavor-preference paradigm by associating flavors with intra-gastric infusion of glucose to minimize the evaluation of the taste of food, we show that retrieval of the post-ingestive nutrient-conditioned flavor memory stimulates multiple brain regions in mice, including the central nucleus of the amygdala (CeA). Moreover, memory retrieval activated the mammalian target of rapamycin complex 1 (mTORC1) in the CeA, while site-specific or systemic inhibition of mTORC1 immediately after retrieval prevented the subsequent expression of the post-ingestive nutrient-associated flavor memory, leading to a long-lasting suppression of reinstatement. Taken together, our findings suggest that the reconsolidation process of a post-ingestive nutrient memory modulates food preferences.

Manipulation of fatty acid composition in a high-fat meal does not result in differential alterations in appetite or food intake in normal weight females: A single-blind randomized crossover study

A behavioral concept that generates a path to obesity is eating in the absence of hunger (EAH). One strategy that may be effective in preventing EAH is the manipulation of dietary fatty acid (FA) composition. However, it remains unclear which FA has the greatest impact on both appetite and EAH. Therefore, the purpose of this study was to examine the effect of different dietary FA compositions (monounsaturated, MUFA; polyunsaturated, PUFA; saturated, SFA) on subjective ratings of appetite and subsequent ad libitum eating after a 3 h postprandial period. Sixteen apparently healthy normal weight females between ages 18-40 completed this randomized, single-blind, crossover study. Participants consumed a HF meal (65% energy from fat) rich in SFA, MUFA, and PUFA with an energy content corresponding to 35% of their measured resting metabolic rate on three separate occasions. Visual analog scales were collected while fasted and every 30 min for 3 h during a postprandial period to measure feelings of hunger, fullness, and desire to eat (DTE). Participants were provided an ad libitum buffet meal 3 h after the HF meal. There were no statistically significant differences for ratings of hunger, fullness, or DTE across conditions. Further, there was no significant difference in energy intake during the ad libitum lunch. We conclude that the manipulation of FA composition in a HF meal does not differentially affect appetite sensations or subsequent energy intake.

Accumbens and amygdala in taste recognition memory: The role of d1 dopamine receptors

The attenuation of taste neophobia (AN) is a good model for studying the structural and neurochemical mechanisms of the emotional component of memory because taste recognition memory exhibits the unique feature of being necessarily linked to hedonic properties. Whilst novel tastes elicit cautious neophobic responses, taste exposures which are not followed by aversive consequences attenuate neophobia as the taste becomes safe and palatable. Given the involvement of the nucleus accumbens in reward and of the amygdala in emotional memories, we applied c-Fos immunohistochemistry as an index of neural activity in Wistar rats that were exposed to a vinegar solution for one, two or six days. An inverse pattern of accumbens nucleus vs amygdala activity was found on the second exposure day on which AN occurred. The number of c-Fos positive cells in the nucleus accumbens shell increased whilst the number of c-Fos positive cells in the basolateral amygdala decreased. Further analyses revealed a positive correlation between AN and the number of c-Fos positive cells in the accumbens shell but a negative correlation in the basolateral amygdala. Furthermore the accumbens-amygdala interplay relevant for AN seems to be mediated by dopamine D1 receptors (D1DR). The injection of SCH23390 (D1DR antagonist) in both the accumbens shell and the basolateral amygdala on the second taste exposure resulted in selectively impaired AN but had opposite long term effects. This finding supports the relevance of a dopaminergic network mediated by D1DRs in the nucleus accumbens shell and basolateral amygdala which is critical for adding the emotional component during the formation of taste memory.

Acquisition and expression of fat-conditioned flavor preferences are differentially affected by NMDA receptor antagonism in BALB/c and SWR mice

Conditioned flavor preferences are elicited by fat (Intralipid) in inbred mouse strains with BALB/c and SWR mice displaying among the most robust preferences. Dopamine D₁ and opioid receptor antagonism differentially reduces the acquisition (learning) and expression (maintenance) of fat-conditioned flavor preferences in these two strains. Because noncompetitive NMDA receptor antagonism with MK-801 differentially altered sugar-conditioned flavor preferences in these strains, and because NMDA receptors are involved in fat intake, the present study examined whether MK-801 differentially altered expression and acquisition of fat (Intralipid)-conditioned flavor preferences in BALB/c and SWR mice. In expression studies, food-restricted male mice alternately consumed a flavored (CS+, e.g., cherry, 5 sessions) 5% Intralipid solution and a differently-flavored (CS-, e.g., grape, 5 sessions) 0.5% Intralipid solution. Two-bottle CS choice tests occurred following vehicle or MK-801 (100, 200µg/kg). MK-801 blocked expression of Intralipid-CFP at both doses in BALB/c mice, but only at the 100µg/kg dose in SWR mice. In acquisition studies, groups of BALB/c (0, 100µg/kg) and SWR (0, 100µg/kg) male mice were treated prior to the ten acquisition training sessions followed by six 2-bottle CS choice tests without injections. MK-801 eliminated acquisition of Intralipid-conditioned flavor preferences in BALB/c mice, and actually changed the preference to an avoidance response in SWR mice. Thus, NMDA receptor signaling appears essential especially for the learning of fat-conditioned flavor preferences in both mouse strains.

Conditioned flavor preferences in animals: Merging pharmacology, brain sites and genetic variance

The elucidation of the behavioral, neurochemical, neuroanatomical and genetic substrates mediating the development of conditioned flavor preferences (CFP) is one of the multi-faceted scientific contributions that Dr. Anthony Sclafani has made to the study of food intake. This review summarizes the results of thirty-five publications over nearly twenty years of collaborations between the Sclafani and Bodnar laboratories. This includes the different approaches employed to study the orosensory (flavor-flavor) and post-ingestive (flavor-nutrient) processes underlying CFP including its acquisition (learning) and expression. It describes how CFP is elicited by different sugars (sucrose, glucose, fructose) and fats (corn oil) in rats, and how strain-specific CFP effects can be observed through the use of inbred mouse strains to evaluate genetic variance. The roles of pharmacological substrates (dopamine, glutamate, opioids, acetylcholine, GABA, cannabinoids) mediating sugar- and fat-CFP acquisition and expression are elucidated. Finally, neuroanatomical sites of action (nucleus accumbens, amygdala, medial prefrontal and orbital frontal cortices, lateral hypothalamus) are evaluated at which dopamine signaling mediates acquisition and expression of different forms of CFP.

Effects of the modern food environment on striatal function, cognition and regulation of ingestive behavior

Emerging evidence from human and animal studies suggest that consumption of palatable foods rich in fat and/or carbohydrates may produce deleterious influences on brain function independently of body weight or metabolic disease. Here we consider two mechanisms by which diet can impact striatal circuits to amplify food cue reactivity and impair inhibitory control. First, we review findings demonstrating that the energetic properties of foods regulate nucleus accumbens food cue reactivity, a demonstrated predictor of weight gain susceptibility, which is then sensitized by chronic consumption of an energy dense diet. Second, we consider evidence for diet-induced adaptations in dorsal striatal dopamine signaling that is associated with impaired inhibitory control and negative outcome learning.

Saccharin Taste Conditions Flavor Preference in Weanling Rats

Innate and learned taste/flavor preferences to chemical stimuli in weanling rats are not fully understood. Our previous study showed that weanling rats could establish conditioned flavor preferences when low, but not high, concentrations of sucrose solutions were used as associative rewarding stimuli. Here, we examined whether 3-week-old rats could acquire flavor learning when the rewarding stimulus was saccharin, a non-nutritive artificial sweetener. In the acquisition session, they consumed water with a flavor (cherry or grape) and 0.1% sodium saccharin with another flavor (grape or cherry) for 15 min daily on alternative days over 6 consecutive days. The subsequent test session revealed significant preferences for the flavor previously associated with saccharin. However, they failed to retain the preference when retested in adulthood at the age of 20 weeks. These behavioral results were similar to those previously demonstrated when 2% sucrose was used as an associative sweetener. Although these 2 solutions were equally preferred, the taste quality may not be the same because the weanling rats showed neophobia to 0.1% saccharin and a larger chorda tympani response than 2% sucrose. The present study showed that a conditioned flavor preference was established to saccharin in weanling rats on the basis of flavor-taste association.

Dopamine receptor signaling in the medial orbital frontal cortex and the acquisition and expression of fructose-conditioned flavor preferences in rats

Systemic dopamine (DA) D1 (SCH23390: SCH) and D2 (raclopride: RAC) antagonists blocked fructose-conditioned flavor preference (CFP) acquisition and expression. Fructose-CFP acquisition was eliminated by medial prefrontal cortex (mPFC) SCH and mPFC or amygdala (AMY) RAC. Fructose-CFP expression was reduced following SCH or RAC in AMY or nucleus accumbens (NAc). The present study examined fructose-CFP acquisition and expression following SCH and RAC in the medial orbital frontal cortex (MOFC), another ventral tegmental area DA target. For fructose-CFP acquisition, five groups of rats received vehicle, SCH (24 or 48 nmol) or RAC (24 or 48 nmol) in the MOFC 0.5h prior to 8 training sessions with one flavor (CS+/Fs) mixed in 8% fructose and 0.2% saccharin, and another flavor (CS-/s) mixed in 0.2% saccharin. In six 2-bottle choice tests in 0.2% saccharin, similar fructose-CFP preferences occurred in groups trained with vehicle (76-77%), SCH24 (69-78%), SCH48 (70-74%) and RAC48 (85-92%). RAC24-trained rats displayed significant CS+ preferences during the first (79%) and third (71%), but not second (58%) test pair. For fructose-CFP expression, rats similarly trained with CS+/Fs and CS- solutions received 2-bottle choice tests following MOFC injections of SCH or RAC (12-48 nmol). CS+ preference expression was significantly reduced by RAC (48 nmol: 58%), but not SCH relative to vehicle (78%). A control group receiving RAC in the dorsolateral prefrontal cortex displayed fructose-CFP expression similar to vehicle. These data demonstrate differential frontal cortical DA mediation of fructose-CFP with mPFC D1 and D2 signaling exclusively mediating acquisition, and MOFC D2 signaling primarily mediating expression.

Roles of NMDA and dopamine D1 and D2 receptors in the acquisition and expression of flavor preferences conditioned by oral glucose in rats

Animals learn to prefer flavors associated with the intake of sugar (sucrose, fructose, glucose) and fat (corn oil: CO) solutions. Conditioned flavor preferences (CFP) have been elicited for sugars based on orosensory (flavor-flavor: e.g., fructose-CFP) and post-ingestive (flavor-nutrient: e.g., intragastric (IG) glucose-CFP) processes. Dopamine (DA) D1, DA D2 and NMDA receptor antagonism differentially eliminate the acquisition and expression of fructose-CFP and IG glucose-CFP. However, pharmacological analysis of fat (CO)-CFP, mediated by both flavor-flavor and flavor-nutrient processes, indicated that acquisition and expression of fat-CFP were minimally affected by systemic DA D1 and D2 antagonists, and were reduced by NMDA antagonism. Therefore, the present study examined whether systemic DA D1 (SCH23390), DA D2 (raclopride) or NMDA (MK-801) receptor antagonists altered acquisition and/or expression of CFP induced by oral glucose that should be mediated by both flavor-flavor and flavor-nutrient processes. Oral glucose-CFP was elicited following by training rats to drink one novel flavor (CS+, e.g., cherry) mixed in 8% glucose and another flavor (CS-, e.g., grape) mixed in 2% glucose. In expression studies, food-restricted rats drank these solutions in one-bottle sessions (2 h) over 10 days. Subsequent two-bottle tests with the CS+ and CS- flavors mixed in 2% glucose occurred 0.5 h after systemic administration of vehicle (VEH), SCH23390 (50-800 nmol/kg), raclopride (50-800 nmol/kg) or MK-801 (50-200 μg/kg). Rats displayed a robust CS+ preference following VEH treatment (94-95%) which was significantly though marginally attenuated by SCH23390 (67-70%), raclopride (77%) or MK-801 (70%) at doses that also markedly reduced overall CS intake. In separate acquisition studies, rats received VEH, SCH23390 (50-400 nmol/kg), raclopride (50-400 nmol/kg) or MK-801 (100 μg/kg) 0.5 h prior to ten 1-bottle training trials with CS+/8%G and CS-/2%G training solutions that was followed by six 2-bottle CS+ vs. CS- tests in 2% glucose conducted without injections. The significant and persistent CS+ preferences observed in the VEH (94-98%) group was significantly reduced by rats receiving SCH23390 at 400 nmol/kg (65-73%), raclopride at 200 or 400 nmol/kg (76-82%) or MK-801 at 100 μg/kg (68-69%). Thus, systemic DA D1 and DA D2 receptor antagonism produced smaller reductions in the expression of oral glucose-CFP relative to fructose-CFP or IG-glucose-CFP. Correspondingly, systemic DA D1, DA D2 and NMDA receptor antagonism also produced smaller reductions in the acquisition of oral glucose-CFP relative to fructose-CFP or IG-glucose-CFP. These data suggest, but do not prove, that the magnitude and persistence of these receptor antagonist effects upon sugar-CFP might depend upon the individual or combined engagement of flavor-flavor and flavor-nutrient processes.

Rapid post-oral stimulation of intake and flavor conditioning in rats by glucose but not a non-metabolizable glucose analog

Mice adapted to drink a flavored saccharin solution (CS-) paired with intragastric (IG) self-infusions of water rapidly increase their intake of a new flavored solution (CS+) that is paired with IG glucose self-infusions. The present study extends this method to examine post-oral glucose appetition in rats. Food-restricted rats were trained to consume a CS- flavor (e.g., grape saccharin) paired with IG water in 5 daily 1-h tests. In the next 3 tests, they drank a CS+ (e.g., cherry saccharin) paired with IG glucose. Rats infused with 8% glucose increased intake significantly on CS+ Test 1, but those infused with 16% glucose showed only a small increase in intake, which may reflect a counteracting satiating effect. Both groups further increased CS+ intakes in Tests 2 and 3, and preferred (81%) the CS+ to the CS- in a two-bottle test without infusions. A second experiment investigated rats' responses to IG alpha-methyl-d-glucopyranoside (MDG), a non-metabolizable sugar analog which stimulates CS+ intake and preference in mice. The rats reduced their intake of the MDG-paired CS+ flavor over sessions, and preferred the CS- to the CS+ in the choice test. The glucose data show that rats, like mice, rapidly detect the sugar's positive post-oral effects that can stimulate intake within the first hour of exposure. The MDG avoidance may indicate a greater sensitivity to its post-oral inhibitory effects in rats than in mice, or perhaps slower clearance of MDG in rats. The test protocol described here can be used to investigate the peripheral and central processes involved in stimulation of intake by post-oral nutrients in rats.

Effect of dopamine D1 and D2 receptor antagonism in the lateral hypothalamus on the expression and acquisition of fructose-conditioned flavor preference in rats

The attraction to sugar-rich foods is influenced by conditioned flavor preferences (CFP) produced by the sweet taste of sugar (flavor-flavor learning) and the sugar's post-oral actions (flavor-nutrient) learning. Brain dopamine (DA) circuits are involved in both types of flavor learning, but to different degrees. This study investigated the role of DA receptors in the lateral hypothalamus (LH) on the flavor-flavor learning produced the sweet taste of fructose. In an acquisition study, food-restricted rats received bilateral LH injections of a DA D1 receptor antagonist (SCH23390), a D2 antagonist (RAC, raclopride) or vehicle prior to 1-bottle training sessions with a flavored 8% fructose+0.2% saccharin solution (CS+/F) and a less-preferred flavored 0.2% saccharin solution (CS-). Drug-free 2-bottle tests were then conducted with the CS+ and CS- flavors presented in saccharin. The fructose-CFP did not differ among groups given vehicle (76%), 12 nmol SCH (78%), 24 nmol (82%) or 24 nmol RAC (90%) during training. In an expression study with rats trained drug-free, LH injections of 12 or 24 nmol SCH or 12-48 nmol RAC prior to 2-bottle tests did not alter CS+ preferences (77-90%) relative to vehicle injection (86%). Only a 48 nmol SCH dose suppressed the CS+ preference (61%). The minimal effect of LH DA receptor antagonism upon fructose flavor-flavor conditioning differs with the ability of LH SCH injections to block the acquisition of glucose flavor-nutrient learning.