Pontine and thalamic influences on fluid rewards: I. Operant responding for sucrose and corn oil

Flavor preferences conditioned by nutritive and non-nutritive sweeteners in mice

Recent studies suggest that preferences are conditioned by nutritive (sucrose) but not by non-nutritive (sucralose) sweeteners in mice. Here we compared the effectiveness of nutritive and non-nutritive sweeteners to condition flavor preferences in three mouse strains. Isopreferred sucrose and sucralose solutions both conditioned flavor preferences in C57BL/6J (B6) mice but sucrose was more effective, consistent with its post-oral appetition action. Subsequent experiments compared flavor conditioning by fructose, which has no post-oral appetition effect in B6 mice, and a sucralose+saccharin mixture (SS) which is highly preferred to fructose in 24-h choice tests. Both sweeteners conditioned flavor preferences but fructose induced stronger preferences than SS. Training B6 mice to drink a flavored SS solution paired with intragastric fructose infusions did not enhance the SS-conditioned preference. Thus, the post-oral nutritive actions of fructose do not explain the sugar's stronger preference conditioning effect. Training B6 mice to drink a flavored fructose solution containing SS did not reduce the sugar-conditioned preference, indicating that SS does not have an off-taste that attenuates conditioning. Although B6 mice strongly preferred flavored SS to flavored fructose in a direct choice test, they preferred the fructose-paired flavor to the SS-paired flavor when these were presented in water. Fructose conditioned a stronger flavor preference than an isopreferred saccharin solution, indicating that sucralose is not responsible for the limited SS conditioning actions. SS is highly preferred by FVB/NJ and CAST/EiJ inbred mice, yet conditioned only weak flavor preferences. It is unclear why highly or equally preferred non-nutritive sweeteners condition weaker preferences than fructose, when all stimulate the same T1r2/T1r3 sweet receptor. Recent findings support the existence of non-T1r2/T1r3 glucose taste sensors; however, there is no evidence for receptors that respond to fructose but not to non-nutritive sweeteners.

Drug-motivated behavior in rats with lesions of the thalamic orosensory area

Rats suppress intake of a palatable taste cue when paired with a rewarding or an aversive stimulus in appetitive or aversive conditioning, respectively. A similar phenomenon occurs with drugs of abuse, but the nature of this conditioning has been subject for debate. While relatively little is known about the underlying neural circuitry, we recently reported bilateral lesions of the thalamic trigeminal orosensory area isolate drug-induced suppression of intake of a taste cue. The lesion blocks avoidance of the taste cue when paired with experimenter delivered drugs of abuse, yet has no effect on avoidance of the same cue when paired with an aversive agent or when it predicts access to a highly palatable sucrose solution. We hypothesize the lesion may blunt the rewarding properties of the drug. To test this, we used a runway apparatus, as running speed has been shown to increase with increasing reward value. Our hypothesis was supported by failure of the lesioned rats to increase running speed for morphine. Interestingly, lesioned rats did avoid intake of the drug-paired cue when presented in the runway apparatus and displayed naloxone-precipitated withdrawal. Using a partial crossover design, the lesion prevented avoidance of a cocaine-paired cue when presented in the home cage. We conclude that the lesion disrupts avoidance of a taste cue in anticipation of the rewarding properties of a drug but, at least in the presence of contextual cues, allows for avoidance of a taste cue as it elicits the onset of an aversive conditioned state of withdrawal.

Alterations in sucrose sham-feeding intake as a function of diet-exposure in rats maintained on calorically dense diets

We previously reported that rats increase meal size upon initial presentation of a calorically dense diet. The increase may be attributed to increased orosensory stimulation and/or reduced sensitivity to post-ingestive inhibitory signals. During feeding both types of signals are simultaneously in play; thus here, we compare responses in rats presented a high-energy diet (HE) or 45% high-fat diet (HF) with those of chow-fed controls (CHOW) in a sham-feeding procedure in which post-ingestive feedback is minimized. Measures of sham-feeding to sucrose were taken before diet manipulation (baseline), ~5 days (dynamic phase) and ~6 weeks (static phase) following introduction of the palatable diet, as well as after animals were switched back to standard chow (recovery phase). Some but not all the hypotheses based on our previous findings were confirmed by the outcomes here. Consistent with our hypothesis that enhanced orosensory stimulation during the dynamic phase compared with the static phase would generalize to increased intake of other palatable stimuli, HE rats showed higher sucrose intake during the dynamic phase compared with the static phase. Contrary to what we hypothesized, HE and HF rats did not increase responses to sucrose compared to CHOW rats. In fact, HE rats showed decreased responses compared to CHOW controls. Thus changes in orosensory stimulation do not necessarily generalize to increased intake of other palatable stimuli.

Gastric bypass in rats does not decrease appetitive behavior towards sweet or fatty fluids despite blunting preferential intake of sugar and fat

After Roux-en-Y gastric bypass surgery (RYGB), patients report consuming fewer fatty and dessert-like foods, and rats display blunted sugar and fat preferences. Here we used a progressive ratio task (PR) in our rat model to explicitly test whether RYGB decreases the willingness of rats to work for very small amounts of preferred sugar- and/or fat-containing fluids. In each of two studies, two groups of rats - one maintained on a high-fat diet (HFD) and standard chow (CHOW) and one given CHOW alone - were trained while water-deprived to work for water or either Ensure or 1.0 M sucrose on increasingly difficult operant schedules. When tested before surgery while nondeprived, HFD rats had lower PR breakpoints (number of operant responses in the last reinforced ratio) for sucrose, but not for Ensure, than CHOW rats. After surgery, at no time did rats given RYGB show lower breakpoints than SHAM rats for Ensure, sucrose, or when 5% Intralipid served postoperatively as the reinforcer. Nevertheless, RYGB rats showed blunted preferences for these caloric fluids versus water in 2-bottle preference tests. Importantly, although the Intralipid and sucrose preferences of RYGB rats decreased further over time, subsequent breakpoints for them were not significantly impacted. Collectively, these data suggest that the observed lower preferences for normally palatable fluids after RYGB in rats may reflect a learned adjustment to altered postingestive feedback rather than a dampening of the reinforcing taste characteristics of such stimuli as measured by the PR task in which postingestive stimulation is negligible.

Effects of SKF-83566 and haloperidol on performance on progressive ratio schedules maintained by sucrose and corn oil reinforcement: quantitative analysis using a new model derived from the Mathematical Principles of Reinforcement (MPR)

RATIONALE

Mathematical models can assist the interpretation of the effects of interventions on schedule-controlled behaviour and help to differentiate between processes that may be confounded in traditional performance measures such as response rate and the breakpoint in progressive ratio (PR) schedules.

OBJECTIVE

The effects of a D1-like dopamine receptor antagonist, 8-bromo-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepin-7-ol hydrobromide (SKF-83566), and a D2-like receptor antagonist, haloperidol, on rats' performance on PR schedules maintained by sucrose and corn oil reinforcers were assessed using a new model derived from Killeen's (Behav Brain Sci 17:105-172, 1994) Mathematical Principles of Reinforcement.

METHOD

Separate groups of rats were trained under a PR schedule using sucrose or corn oil reinforcers. SKF-83566 (0.015 and 0.03 mg kg(-1)) and haloperidol (0.05 and 0.1 mg kg(-1)) were administered intraperitoneally (five administrations of each treatment). Running and overall response rates in successive ratios were analysed using the new model, and estimates of the model's parameters were compared between treatments.

RESULTS

Haloperidol reduced a (the parameter expressing incentive value) in the case of both reinforcers, but did not affect the parameters related to response time and post-reinforcement pausing. SKF-83566 reduced a and k (the parameter expressing sensitivity of post-reinforcement pausing to the prior inter-reinforcement interval) in the case of sucrose, but did not affect any of the parameters in the case of corn oil.

CONCLUSIONS

The results are consistent with the hypothesis that blockade of both D1-like and D2-like receptors reduces the incentive value of sucrose, whereas the incentive value of corn oil is more sensitive to blockade of D2-like than D1-like receptors.

Bilateral lesions of the thalamic trigeminal orosensory area dissociate natural from drug reward in contrast paradigms

Substance abuse and addiction are associated with an apparent devaluation of, and inattention to, natural rewards. This consequence of addiction can be modeled using a reward comparison paradigm where rats avoid intake of a palatable taste cue that comes to predict access to a drug of abuse. Evidence suggests rats avoid intake following such pairings, at least in part, because the taste cue pales in comparison to the highly rewarding drug expected in the near future. In accordance, lesions of the gustatory thalamus or cortex eliminate avoidance of a taste cue when paired with either a drug of abuse or a rewarding sucrose solution, but not when paired with the aversive agent, LiCl. The present study used bilateral ibotenic acid lesions to evaluate the role of a neighboring thalamic structure, the trigeminal orosensory area (TOA), in avoidance of a gustatory cue when paired with sucrose (experiment 1), morphine (experiment 2), cocaine (experiment 3), or LiCl (experiment 4). The results show that the TOA lesion disrupts, but does not eliminate avoidance of a taste cue that predicts access to a preferred sucrose solution and leaves intact the development of a LiCl-induced conditioned taste aversion. The lesion does, however, eliminate the suppression of intake of a taste cue when paired with experimenter-administered morphine or cocaine using our standard parameters. As such, this is the first manipulation found to dissociate avoidance of a taste cue when mediated by a sweet or by a drug of abuse.

Pontine and thalamic influences on fluid rewards: II. Sucrose and corn oil conditioned aversions

In this study conditioned aversions were produced in sham feeding rats to limit postingestive feedback from the oral stimulus. All control rats learned an aversion to either 100% corn oil or 0.3 M sucrose when ingestion of these stimuli was followed by an injection of lithium chloride (LiCl). Rats with lesions of the ventroposteromedial thalamus also learned to avoid either corn oil or sucrose. After 3 trials, rats with damage to the parabrachial nuclei (PBN) learned to avoid 100% corn oil, but failed to do so when the stimulus was 0.3 M sucrose. These results support our hypothesis that the PBN is necessary to appropriately respond to a taste, but not an oil cue as a function of experience (i.e., pairings with LiCl). The results also are consistent with our results from operant tasks demonstrating that the trigeminal thalamus, the ventroposteromedial nucleus, is not required for responding to the rewarding properties of sucrose, oil, or for modifying the response to these stimuli as a function of experience.

Pontine and thalamic influences on fluid rewards: III. Anticipatory contrast for sucrose and corn oil

An anticipatory contrast effect (ACE) occurs when, across daily trials, an animal comes to respond less than normally to a first stimulus when it is followed shortly by a second, more preferred solution. Classically, ACE is studied using a low (L) concentration of saccharin or sucrose, followed by access to a higher (H) concentration of sucrose. Subjects in the control condition have two bouts of access to the weaker solution presented on the same schedule. The ACE is measured by the difference in intake of the first bout low solution between subjects in the low-low (L-L) vs. the low-high (L-H) conditions. Here we used this paradigm with sham feeding rats and determined that nutritional feedback was unnecessary for the development of ACE with two concentrations of sucrose or with two concentrations of corn oil. Next we showed that ibotenic acid lesions centered in the orosensory thalamus spared ACEs for both sucrose and corn oil. In contrast, lesions of the pontine parabrachial nuclei (PBN), the second central relay for taste in the rat, disrupted ACEs for both sucrose and corn oil. Although the sensory modalities needed for the oral detection of fats remain controversial, it appears that the PBN is involved in processing the comparison of disparate concentrations of sucrose and oil reward.