Conditioned Flavor Aversion Research Articles

Assessing the inhibitory properties of a latent inhibitor in flavor-aversion learning

In Experiment 1, rats received 16 nonreinforced trials of exposure to a flavor (A) that was subsequently used as the conditioned stimulus in flavor-aversion conditioning. In the critical condition, Flavor A was presented in compound with a different novel flavor on each of the eight daily trials. This treatment produced latent inhibition, in that this preexposure retarded conditioning just as did 16 trials with A alone. Rats in the control conditions, given no preexposure or exposure just to the sequence of novel flavors, learned readily. Experiment 2 examined the effects of these forms of preexposure on performance on a summation test, in which Flavor A was presented in compound with a separately conditioned flavor (X). The preexposure procedure in which A was presented along with novel flavors rendered A effective in inhibiting the response conditioned to X on that test. The conclusion, that this form of training can establish the target stimulus as a conditioned inhibitor, is predicted by the account of latent inhibition put forward by Hall and Rodríguez (2010) which proposes that the latent inhibition effect is a consequence both of a reduction in the associability of the stimulus and of a process of inhibitory associative learning that opposes the initial expectation that a novel event will be followed by some consequence.

Food avoidance learning based on voluntary wheel running in laboratory mice (Mus musculus)

Mice show a reluctance to eat unfamiliar food, when they first encounter it. This neophobic reaction is conventionally habituated by repeated trials: the mice gradually increase their consumption of the novel food. The new finding reported here is that the consumption remains low in mice that voluntarily run in activity wheels after the novel food access. This effect implies that running yields Pavlovian conditioned flavor aversion, which suppresses, otherwise increasing, consumption of the novel food. In the present research, the effect was demonstrated with a between-group design by pitting experimental mice receiving cheese-running paired treatment against cheese/running unpaired control mice (Experiment 1). The running-based food avoidance in mice was also shown in a differential conditioning paradigm, where one of two novel snacks (chocolate and marshmallow) was paired with running while the other was not, in non-deprived animals (Experiment 2 A) and food-deprived animals (Experiment 2B). These results concord with those previously reported in rats, indicating the generality of the phenomenon.

Social and anxiety-like behaviors contribute to nicotine self-administration in adolescent outbred rats

Both emotional and social traits interact with genetic factors to influence smoking behavior. We previously established a socially acquired nicotine intravenous self-administration model where social learning of a nicotine-associated odor cue reversed conditioned flavor aversion and promoted nicotine intake. In this study, we first phenotyped ~800 adolescent heterogeneous stock rats in open field, novel object interaction, social interaction, elevated plus maze, and marble burying behaviors. These rats were then phenotyped on socially acquired nicotine self-administration. We found 243 significant correlations between different behavioral tests. Principal component regression analysis found that ~10–20% of the variance in nicotine-related measures, such as intake during the first or the last three fixed-ratio sessions, the progressive ratio session, and reinstatement behavior, can be explained by variations in behavioral traits. Factors corresponding to social behavior and anxiety were among the strongest predictors of nicotine intake and reinstatement of nicotine-seeking behavior. We also found many sex differences in behavioral measures. These data indicated that the genetic diversity of this population, in combination with social behaviour and anxiety, are significant contributors to the divergent nicotine self-administration behavior and indicated a high probability of discovering sex-specific genetic mechanisms for nicotine intake in future genome-wide association studies.

Social learning promotes nicotine self-administration by facilitating the extinction of conditioned aversion in isogenic strains of rats

Both social environment and genetic factors are critical for smoking initiation and nicotine addiction. We reported that rats developed conditioned flavor (i.e., taste and odor) aversion to intravenously self-administered (IVSA) nicotine, and that social learning promoted nicotine IVSA with flavor cues. We thus tested the hypothesis that socially acquired nicotine IVSA is a heritable trait by using female rats of six inbred strains and six F1 hybrids. Each strain was tested for 10 daily IVSA sessions. We found that the intake of nicotine (15 and 30 μg/kg/inf) varied among these strains by 33.7–56.6-fold. The heritability of nicotine intake was estimated to be 0.54–0.65. Further, there was a strong correlation in nicotine intake (R2 = 0.85, p < 0.0001) between the two nicotine doses. Another cohort of rats was given three daily IVSA sessions followed by five sessions that tested conditioned flavor aversion. Nicotine intake was highly correlated with the extinction of the conditioned aversion (R2 = 0.58, p < 0.005). These data showed that nicotine intake in the socially acquired nicotine self-administration model is controlled by genetic factors and that the role of social learning is likely in facilitating the extinction of conditioned aversive response to nicotine.

Increasing trial number produces augmentation, not blocking, in flavor-aversion conditioning

Typically, A+→AX+ conditioning yields significantly weaker responding to CS X, a phenomenon known as blocking. Yet, in flavor-aversion conditioning, A+→AX+ conditioning yields a significantly stronger response to X, termed augmentation. Two flavor-aversion experiments with rat subjects and rotationally induced illness were conducted to determine if trial number was a factor that affected the expression of augmentation. An increase in the number of A+ preconditioning trials resulted in a stronger augmented aversion (Experiment 1). Similarly, an increase in the number of AX+ compound conditioning trials also produced a significantly stronger augmentation effect (Experiment 2). Therefore, these data show that variations in trial number are not the determining factor in the expression of augmentation or blocking, and they confirm that the strength of augmented aversions are directly related to the strength of the aversion of the preconditioned cue. Finally, these results are the first to show odor-aversion conditioning and augmented taste aversions produced via rotational stimulation.

Within-compound associations mediate augmented flavor aversions

Five experiments with rat subjects explored flavor-aversion conditioning in the A+/AX+ design. In Experiment 1, odor preconditioning prior to taste+odor compound conditioning significantly strengthened a taste aversion, and taste preconditioning prior to taste+odor conditioning significantly augmented an odor aversion. Experiments 2A and 2B eliminated the alternative explanation that the augmentation effect was produced by differential exposure to the compound during conditioning. Next, we examined a possible mechanism of these augmentation effects by employing post-conditioning extinction of the augmenting stimulus (CS A); previous research had also used this technique to explore augmentation, but those studies yielded conflicting results. In both Experiments 3A and 3B, post-conditioning extinction of CS A led to a significant decrease in the strength of the augmented aversion to CS X, regardless of stimulus modality or stimulus salience. Collectively, these results suggest augmented flavor aversions are produced via within-compound associations between CS A and CS X.

Effects of dietary choline availability on latent inhibition of flavor aversion learning

ObjectiveIt has been previously reported that dietary choline supplementation might affect latent inhibition (LI) using a conditioned suppression procedure in rats. We have assessed the effect of dietary choline on LI of flavor aversion learning.MethodAdult male Wistar rats received a choline supplemented (5 g/kg), deficient (0 g/kg), or standard (1.1 g/kg) diet for 3 months. After this supplementation period, all rats went through a conditioned taste aversion (CTA) procedure, half of them being pre-exposed to the conditioned stimulus before the conditioning.ResultsThe results indicated that choline deficiency prevents LI of conditioned flavor aversion to cider vinegar (3%) induced by a LiCl (0.15 M; 2% body weight) intraperitoneal injection, while choline supplementation enhances CTA leading to slower extinction.DiscussionThe role of the brain systems modulating attentional processes is discussed.

Effects of extended context discrimination training and context extinction on transfer of context dependency of conditioned flavor aversion

We trained rats in a context discrimination paradigm by pairing a sucrose solution with lithium chloride in one context (conditioning context) and simple exposure to the same fluid in a second (neutral) context to establish a context-dependent aversion to the conditioned fluid. We then investigated whether transfer of the context dependency to a test fluid (a sodium chloride solution) was affected by two post-discrimination training treatments, an extended context discrimination training, and non-reinforced exposure to the conditioning context (context extinction). We found that the context-dependent flavor aversion that had been specific to sucrose transferred to the test fluid after the extensive training (Experiment 1). Context extinction eliminated the transfer effect that had been observed immediately after the context discrimination training (Experiment 2). In addition, an aversion acquired by sucrose through a simple conditioning of sucrose–LiCl pairings did not generalize to the test fluid (Experiment 3). These results emphasize the importance of a Pavlovian excitatory association between the conditioning context and nausea as a primary source of transfer of the context dependency, rather than a generalization of aversion acquired by the conditioned fluid to the test fluid.

Flavor learning in weanling rats and its retention

The present study examined whether weanling animals can acquire associative memory for reward and retain it several weeks later. Three-week-old Wistar male rats were trained in a flavor learning task. Half of the rats received unsweetened grape-flavored water on odd-numbered days and sweetened (sucrose) cherry-flavored solution on even-numbered days. The remaining rats received sweetened grape-flavored solution on odd-numbered days and unsweetened cherry-flavored water on even-numbered days. During the acquisition session, the liquid was presented to each rat for 15min daily for 6 consecutive days. In the following test session, each rat was presented with unsweetened cherry-flavored water and grape-flavored water simultaneously for 15min daily for 4 consecutive days. The rats showed significant preferences for the flavor previously associated with 2% and 10% sucrose, significant aversion to the flavor associated with 30% sucrose, and no particular preference or aversion to the flavor associated with 20% sucrose, indicating a hedonic shift from positive to negative with an increasing concentration of sucrose. The association learning acquired at the age of 3weeks was retained when re-tested in adulthood at the age of 20weeks. In contrast to the conditioned flavor aversion associated with 30% sucrose, 20-week-old rats showed a preference for this flavor. In accordance with these learning effects, 3-week-old rats preferred 2% sucrose to 30% sucrose, and the reverse was true in 20-week-old rats. The reasons for rejection of high-concentration sucrose by weanling rats are also discussed. The present study showed that weanling rats established a conditioned flavor preference or aversion depending on the concentration of associated sucrose and retained it in adulthood, indicating that feeding experience in the weanling period is important in influencing later dietary preferences.

A Conditioned Aversion Study of Sucrose and SC45647 Taste in TRPM5 Knockout Mice

Previously, published studies have reported mixed results regarding the role of the TRPM5 cation channel in signaling sweet taste by taste sensory cells. Some studies have reported a complete loss of sweet taste preference in TRPM5 knockout (KO) mice, whereas others have reported only a partial loss of sweet taste preference. This study reports the results of conditioned aversion studies designed to motivate wild-type (WT) and KO mice to respond to sweet substances. In conditioned taste aversion experiments, WT mice showed nearly complete LiCl-induced response suppression to sucrose and SC45647. In contrast, TRPM5 KO mice showed a much smaller conditioned aversion to either sweet substance, suggesting a compromised, but not absent, ability to detect sweet taste. A subsequent conditioned flavor aversion experiment was conducted to determine if TRPM5 KO mice were impaired in their ability to learn a conditioned aversion. In this experiment, KO and WT mice were conditioned to a mixture of SC45647 and amyl acetate (an odor cue). Although WT mice avoided both components of the stimulus mixture, they avoided SC45647 more than the odor cue. The KO mice also avoided both stimuli, but they avoided the odor component more than SC45647, suggesting that while the KO mice are capable of learning an aversion, to them the odor cue was more salient than the taste cue. Collectively, these findings suggest the TRPM5 KO mice have some residual ability to detect SC45647 and sucrose, and, like bitter, there may be a TRPM5-independent transduction pathway for detecting these substances.

Systemic 5-bromo-2-deoxyuridine induces conditioned flavor aversion and c-Fos in the visceral neuraxis.

5-bromo-2-deoxyuridine (BrdU) is often used in studies of adult neurogenesis and olfactory learning, but it can also have toxic effects on highly proliferative tissue. We found that pairing Kool-Aid flavors with acute systemic injections of BrdU induced strong conditioned flavor aversions. Intermittent injections during Kool-Aid-glucose conditioning interfered with learning of a conditioned flavor-nutrient preference. Acute injection of BrdU also elevated plasma corticosterone levels and induced c-Fos in the visceral neuraxis. Thus, acute or intermittent systemic injections of BrdU (50-200 mg/kg) have aversive effects that may interfere with learning.

Systemic BrdU induces conditioned flavor aversions and c-Fos

Systemic BrdU induces conditioned flavor aversions and c-Fos

Dynamics of the Volatile Defense of Winter “Dormant” Balsam Poplar (Populus balsamifera)

6-Hydroxycylohex-2-en-1-one (6-HCH) has been reported as a major chemical defense of the winter-dormant internodes of balsam poplar (Populus balsamifera) against feeding by herbivores such as the snowshoe hare (Lepus americanus). We report that the concentration of 6-HCH in the fall internodes is triggered by a single hard frost, and then undergoes an exponential decline through volatilization over the winter that results in barely detectable quantities by early spring. We conclude that the role of 6-HCH in the defense of mature balsam poplar is more complex than simply acting as a toxin. Rather, 6-HCH's role as a defensive agent must evolve over the course of the winter from being a co-toxin to a cue for a conditioned flavor aversion (CFA) to finally having no role by late spring.

Single-trial exercise-induced taste and odor aversion learning in humans

In the present study, it was investigated whether humans acquire an aversion for a flavor paired with a single bout of exercise, and if so, to what degree this effect requires the experience of gastrointestinal distress. To this end, a total of 58 participants either consumed or merely tasted a specifically flavored solution directly prior to a 30 min running exercise. In both cases this led to a negative shift in subjective liking of the flavor (taste and odor) in comparison to the evaluation of another flavor not explicitly paired with exercise, indicative of a conditioned flavor aversion. The degree of subjectively experienced exercise-related gastrointestinal distress did not predict this negative hedonic shift for the flavor paired with the running exercise, implying that such distress may not be a prerequisite for exercise-induced flavor aversion learning in humans.

Factors determining the effects of associative activation on habituation.

In 2 experiments, rats received flavor-aversion conditioning with two flavors, B and C, to which they had been preexposed. In both experiments, C was preexposed in compound with another flavor in a block of CX trials. In Experiment 1, B was presented in compound with Y, and BY trials were alternated with presentations of Y alone. In Experiment 2, B was presented in compound with X, and BX trials were alternated with presentations of X alone. No difference was detected in Experiment 1 between B and C in the ease with which they conditioned, but in Experiment 2 it was found that B conditioned more readily than C. This latter result is consistent with the hypothesis that experience with the associate of a target stimulus can act to maintain the effective salience of that stimulus; however, the results of Experiment 1 challenge this interpretation or indicate the operation of other factors that limit the effectiveness of this salience modulation process.

Conditioned flavor aversion and brain Fos expression following exposure to arsenic

Recent advances in the knowledge of the cellular effects of arsenic have provided insights into the molecular mechanisms of arsenic-associated carcinogenesis, immunotoxicity and cardiovascular disease. In the present experiments we tested the hypothesis that the arrival of arsenic to the gastrointestinal (GI) tract is detected by the gut–brain axis, which includes hindbrain and forebrain nuclei activated by GI stimulation. As a marker of neuronal activation we measured Fos expression using immunohistochemistry. Because Fos expression in these nuclei is closely linked to the development of conditioned flavor aversion (CFA) we also tested the effect of arsenic on CFA. Our experiments indicate that arsenic ingestion is readily detected by the brain, as shown by increased Fos expression after oral administration of arsenic. Furthermore, the vagus nerve, which supplies information from the GI tract to the brain, is not involved in this response because a complete subdiaphragmatic vagotomy did not reduce the effect of arsenic on brain Fos expression, but enhanced this response. In parallel, arsenic ingestion is associated with a robust, dose-dependent CFA, which started at doses as low as 0.1 mg/kg body weight. In summary, these data indicate that arsenic given by oral administration is detected by the brain in low concentrations, and activates specific nuclei, which might trigger behavioral responses, such as CFA.

Lateral Parabrachial Lesions Disrupt Paraoxon-Induced Conditioned Flavor Avoidance

Preliminary clinical evidence obtained in Gulf War veterans and patients suffering multiple chemical sensitivity points to the existence of a potential link between environmental exposure to organosphosphates (OPs) and the emergence of unspecific sickness syndromes in which associative Pavlovian conditioning might be partly involved. A laboratory animal model might be a useful tool for analyzing the involvement of conditioning in sickness syndromes potentially linked to OP poisoning. The first objective in the present study was to determine if paraoxon (PX), the neuroactive metabolite of the OP parathion, elicits a conditioned avoidance response to a novel stimulus (a taste-odor compound) in a conditioned flavor aversion procedure. Data obtained in Experiment 1 show conditioned flavor avoidance, demonstrative of the associative nature of the sickness properties of PX. The second objective was to characterize the nature of the specific physiological cue serving as the unconditioned stimulus in PX-induced conditioned avoidance. Despite PX administration did induce cholinergic hyperactivity, as measured by body hypothermia and increased jaw movements, lesions of the lateral parabrachial area (lPB) disrupted PX-elicited flavor avoidance responses, indicating that cholinergic signs were not sufficient as unconditioned stimuli supporting avoidance responses. Given that lPB neural integrity is necessary to process aversive interoceptive information, disruption of conditioned flavor avoidance as a result of lPB lesions is consistent with a central interruption of interoceptive processing in PX-poisoned animals. Data are discussed under the light of the hypothesis claiming the importance of associative processes and noncholinesterase targets in sickness syndromes potentially induced by OP exposure.

Marker-Based Quantitative Genetics in the Wild?: The Heritability and Genetic Correlation of Chemical Defenses in Eucalyptus

Marker-based methods for estimating heritability and genetic correlation in the wild have attracted interest because traditional methods may be impractical or introduce bias via G x E effects, mating system variation, and sampling effects. However, they have not been widely used, especially in plants. A regression-based approach, which uses a continuous measure of genetic relatedness, promises to be particularly appropriate for use in plants with mixed-mating systems and overlapping generations. Using this method, we found significant narrow-sense heritability of foliar defense chemicals in a natural population of Eucalyptus melliodora. We also demonstrated a genetic basis for the phenotypic correlation underlying an ecological example of conditioned flavor aversion involving different biosynthetic pathways. Our results revealed that heritability estimates depend on the spatial scale of the analysis in a way that offers insight into the distribution of genetic and environmental variance. This study is the first to successfully use a marker-based method to measure quantitative genetic parameters in a tree. We suggest that this method will prove to be a useful tool in other studies and offer some recommendations for future applications of the method.

Phenotype-based treatment of dietary obesity: differential effects of fenofibrate in obesity-prone and obesity-resistant rats

High-fat diets (HFDs) promote hyperphagia and adiposity in animals and human beings. To test the hypothesis that limitations on fat oxidation underlie this propensity for diet-induced obesity, rats were treated with fenofibrate, which enhances fat oxidation mainly in liver by inducing expression of enzymes and proliferation of organelles involved in fatty acid oxidation. Male Sprague-Dawley rats were fed a HFD (42% fat calorie) for 2 weeks. Rats ranked in the top and bottom thirds for weight gain during this feeding period were designated as obesity prone (OP) and obesity resistant (OR), respectively. Fenofibrate was added to the HFD (0.025% wt/wt) for half of the OP and OR rats. During the next 10 days, fenofibrate treatment significantly ( P < .05) reduced food intake, weight gain, feed efficiency, and adiposity in OP rats to levels seen in control OR rats, but had no such effects in OR rats. Fenofibrate treatment increased whole-body fatty acid oxidation, and in liver, the expression of carnitine palmitoyl transferase I only in OP rats, but enhanced expression of acyl-CoA oxidase in both OP and OR rats. Restricting food intake of OP rats to levels seen in rats given fenofibrate similarly reduced weight gain but had little effect on weight of fat pads. Treatment with the daily dosage of fenofibrate given as a bolus did not produce a conditioned flavor aversion. These results suggest that enhancement of mitochondrial fatty acid oxidation in liver may be an effective phenotype-based treatment strategy for dietary obesity.

Conditioned Flavor Aversions: Assessment of Drug‐Induced Suppression of Food Intake

Administration of a drug following ingestion of a novel food or solution often suppresses subsequent intake of the new food or solution. This suppression is associative, in that consumption is not suppressed when there is no temporal relationship between consumption and drug administration. The robust nature of aversion learning has made this procedure a sensitive and widely used behavioral index of drug side effects. The procedures described in this unit are suitable for work with rodents, and may require modifications, e.g., in presentation of the ingesta and drug for other species. Familiar and novel foods may be used instead of solutions, with similar results.