Rats' learned preferences for flavors encountered early or late in a meal paired with the postingestive effects of glucose

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.

Development of leptin resistance in sucrose drinking rats is associated with consuming carbohydrate-containing solutions and not calorie-free sweet solution

Rats offered 30% sucrose solution plus chow or a sucrose-free diet develop leptin resistance within 4 weeks. This experiment tested whether leptin resistance was associated with the reward of sweet taste or the pre- or post-absorptive effects of consumption of simple carbohydrate. Male Sprague Dawley rats were offered a sucrose-free diet (NS), a diet containing 67% calories as sucrose (HS) or NS diet plus 30% sucrose (LS), 0.03% saccharin (Sacc) or 20% SolCarb^® solution for 38 days. SolCarb^® is a maltodextrin powder. Sacc rats initially drank more than LS rats, but intakes were the same after Day 20. SolCarb^® and LS rats drank the same number of calories from Day 15 to the end of the experiment. SolCarb^® and LS rats ate less dry food than other groups, but total energy intake was greater than that of NS, HS and Sacc groups and over 80% of their energy intake was from carbohydrate. Leptin responsiveness was tested on Days 27 and 32 with each rat acting as its own control. An i.p. injection of 2 mg/kg leptin inhibited food intake of NS, HS and Sacc rats, but had no effect on energy intake of LS or SolCarb^® rats or on consumption of Sacc, sucrose or SolCarb^® solution. At the end of the experiment all of the rats were insulin sensitive, had the same body composition and serum leptin concentrations. These data indicate that consumption of a calorie containing carbohydrate solution and not sweet taste drives the development of leptin resistance and suggest that there is lower threshold for inhibition of hunger than for inhibition of reward by leptin.

Feminizing Adrenocortical Carcinoma with Distinct Histopathological Findings

We herein present a 60-year-old man with adrenocortical carcinoma who had gynecomastia. An endocrinological examination revealed increased levels of serum estradiol and dehydroepiandrosterone-sulfate (DHEA-S) and reduced levels of free testosterone. Magnetic resonance imaging showed an adrenal tumor with heterogeneous intensity. Iodine-131 adosterol scintigraphy showed an increased uptake at the same site. Because feminizing adrenocortical carcinoma was suspected, right adrenalectomy was performed; the pathological diagnosis was adrenocortical carcinoma. The results of immunostaining indicated a virilizing tumor. Aromatase activity was identified on RT-PCR. As disorganized steroidogenesis is pathologically present in adrenocortical carcinoma, this diagnosis should be made with caution.

Enhanced flavor-nutrient conditioning in obese rats on a high-fat, high-carbohydrate choice diet

Through flavor-nutrient conditioning rats learn to prefer and increase their intake of flavors paired with rewarding, postingestive nutritional consequences. Since obesity is linked to altered experience of food reward and to perturbations of nutrient sensing, we investigated flavor-nutrient learning in rats made obese using a high fat/high carbohydrate (HFHC) choice model of diet-induced obesity (ad libitum lard and maltodextrin solution plus standard rodent chow). Forty rats were maintained on HFHC to induce substantial weight gain, and 20 were maintained on chow only (CON). Among HFHC rats, individual differences in propensity to weight gain were studied by comparing those with the highest proportional weight gain (obesity prone, OP) to those with the lowest (obesity resistant, OR). Sensitivity to postingestive food reward was tested in a flavor-nutrient conditioning protocol. To measure initial, within-meal stimulation of flavor acceptance by post-oral nutrient sensing, first, in sessions 1-3, baseline licking was measured while rats consumed grape- or cherry-flavored saccharin accompanied by intragastric (IG) water infusion. Then, in the next three test sessions they received the opposite flavor paired with 5 ml of IG 12% glucose. Finally, after additional sessions alternating between the two flavor-infusion contingencies, preference was measured in a two-bottle choice between the flavors without IG infusions. HFHC-OP rats showed stronger initial enhancement of intake in the first glucose infusion sessions than CON or HFHC-OR rats. OP rats also most strongly preferred the glucose-paired flavor in the two-bottle choice. These differences between OP versus OR and CON rats suggest that obesity is linked to responsiveness to postoral nutrient reward, consistent with the view that flavor-nutrient learning perpetuates overeating in obesity.

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.

Motivational control of latent inhibition in flavor preference conditioning

In three experiments, rats given pairings of a neutral flavor with sucrose showed a preference for that flavor when subsequently allowed to choose between it and water. Preexposure to the flavor produced a latent inhibition effect (reduced the size of the preference) when the rats were hungry during the test (Experiments 1 and 2). Rats that were not hungry during the test failed to show latent inhibition (Experiments 1-3). Experiment 3 confirmed that sucrose-flavor pairings were capable of producing a preference even in nonhungry rats. It is argued that the preference shown by rats that are hungry on test depends on a flavor-nutrient association, a form of learning that is susceptible to latent inhibition in the same way as standard conditioning procedures are. The failure to obtain latent inhibition in nonhungry rats suggests that the preference obtained in these conditions depends on a different form of learning that is less susceptible to the effects of stimulus exposure.

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.