Extinction of a saccharin—lithium association: Assessment by consumption and taste reactivity

Partial reinforcement and conditioned taste aversion: No evidence for resistance to extinction

The partial reinforcement extinction effect (PREE) is the observation that, following training in which a response is followed by reward on only a subset of trials, the response is more resistant to extinction following the total removal of reward than it is after training in which reward is presented on all trials. The PREE is almost ubiquitous in instrumental conditioning procedures but only inconsistently observed in Pavlovian conditioning. In his classic review of animal learning, Mackintosh attributes the bulk of the PREE to generalisation decrement relating to the fact that partial reinforcement typically ensures that acquisition of responding has taken place in conditions similar to that of extinction (e.g., in the absence of the reinforcer). We report here that extinction of a conditioned taste aversion is not retarded by partial reinforcement in terms of either consumption of the taste or hedonic reactions to it (assessed through the analysis of licking microstructure). These results are consistent with Mackintosh’s analysis of the PREE and the way in which it might differ between instrumental and Pavlovian conditioning.

Inulin fiber dose-dependently modulates energy balance, glucose tolerance, gut microbiota, hormones and diet preference in high-fat-fed male rats

Inulin, a popular prebiotic fiber, has been reported to promote satiety and fat loss; however, the dose-response effects of inulin on energy balance and diet preference, and whether the metabolic effects are independent of calorie restriction are not well characterized. Therefore, we compared the effects of diets varying in inulin concentrations on food intake, energy expenditure, body composition, gut microbiota and hormones, and assessed whether inulin-induced hypophagia was due to reduced diet preference. In experiment 1, male rats were randomized to six high-fat diet groups: control (CON, 0% inulin), 2.5% inulin (2.5IN), 10% inulin (10IN), 25% inulin (25IN), 25% cellulose (25CE) or pair-fed to 25IN (25PF) for 21 days. We demonstrate that inulin dose-dependently decreased caloric intake and respiratory quotient; improved glucose tolerance; increased the abundance of Bacteroidetes and Bifidobacterium spp.; decreased Clostridium clusters I and IV; increased butyryl-CoA:acetate CoA-transferase in cecum; upregulated peptide YY, cholecystokinin and proglucagon transcripts in the cecum and colon; and increased plasma peptide YY and glucagon-like peptide-1 concentrations. Importantly, unlike 25PF, 25IN attenuated the reduction in energy expenditure associated with calorie restriction and decreased adiposity. In experiment 2, following four training periods, diet preferences were determined. Although 10IN and 25IN decreased caloric intake, and 25CE increased caloric intake, during training, all high-fiber diets were less preferred. Taken together, this work demonstrates that inulin dose-dependently decreased caloric intake, modulated gut microbiota and upregulated satiety hormones, with metabolic effects being largely independent of caloric restriction.

Nonreinforced flavor exposure attenuates the effects of conditioned taste aversion on both flavor consumption and cue palatability

Nonreinforced exposure to a cue tends to attenuate subsequent conditioning with that cue-an effect referred to as latent inhibition (LI). In the two experiments reported here, we examined LI effects in the context of conditioned taste aversion by examining both the amount of consumption and the microstructure of the consummatory behavior (in terms of the mean size of lick clusters). The latter measure can be taken to reflect affective responses to, or the palatability of, the solution being consumed. In both experiments, exposure to a to-be-conditioned flavor prior to pairing the flavor with nausea produced by lithium chloride attenuated both the reduction in consumption and the reduction in lick cluster sizes typically produced by taste aversion learning. In addition, we observed a tendency (especially in the lick cluster measure) for nonreinforced exposure to reduce neophobic responses to the test flavors. Taken together, these results reinforce the suggestion from previous experiments using taste reactivity methods that LI attenuates the effects of taste aversion on both consumption and cue palatability. The present results also support the suggestion that the failure in previous studies to see concurrent LI effects on consumption and palatability was due to a context specificity produced by the oral taste infusion methods required for taste reactivity analyses. Finally, the fact that the pattern of extinction of conditioned changes in consumption and in lick cluster sizes was not affected by preexposure to the cue flavors suggests that LI influenced the quantity but not the quality of conditioned taste aversion.

Disgust and behavioral avoidance in colorectal cancer screening and treatment: a systematic review and research agenda

BACKGROUND

The emotion of disgust appears to promote psychological and behavioral avoidance, a dynamic that has significant implications in physical and psychological outcomes in colorectal cancer (CRC). Patients, caregivers, and health professionals alike are all potentially susceptible to responding with disgust and the associated avoidance.

OBJECTIVE

This article aimed to review the early-stage literature related to disgust and CRC, consider the clinical implications, and suggest an appropriate research agenda.

METHODS

Given limited research in this area, a systematic review of the literature was broadened to include disgust and all cancers. MEDLINE, Web of Science, SCOPUS, and ProQuest Dissertations and Theses databases were searched, with additional works sourced by reviewing citation lists and/or by contacting the lead authors.

RESULTS

Nine studies were identified relating to disgust and cancer screening, and 6 related to disgust and cancer treatment. Two broad findings emerged: (1) disgust appears to be promoting aversion to (and avoidance of) CRC screening, and (2) several known elicitors of disgust are widely apparent in CRC contexts.

CONCLUSIONS

Disgust likely represents a key emotional substrate for avoidance among CRC patients, caregivers, and health professionals. Further research is required to identify disgust's elicitors and effects in CRC contexts, informing interventions that target early identification of persons at risk of maladaptive outcomes. Exposure therapies and mindfulness training may be well suited to treating disgust-generated avoidance.

IMPLICATIONS FOR PRACTICE

Disgust has significant implications in CRC contexts. Oncology nurses are uniquely positioned to guide clinical interventions and ultimately improve outcomes in this area.

Palatable food avoidance and acceptance learning with different stressors in female rats

Stress activates the hypothalamus-pituitary-adrenal (HPA) axis leading to the release of glucocorticoids (GC). Increased activity of the HPA axis and GC exposure has been suggested to facilitate the development of obesity and metabolic syndrome. Nonetheless, different stressors can produce distinct effects on food intake and may support different directions of food learning e.g. avoidance or acceptance. This study examined whether interoceptive (LiCl and exendin-4) and restraint stress (RS) support similar or distinct food learning. Female rats were exposed to different stressors after their consumption of a palatable food (butter icing). After four palatable food-stress pairings, distinct intakes of the butter icing were observed in rats treated with different stressors. Rats that received butter icing followed by intraperitoneal injections of LiCl (42.3mg/kg) and exendin-4 (10μg/kg) completely avoided the palatable food with subsequent presentations. In contrast, rats experiencing RS paired with the palatable food increased their consumption of butter icing across trials and did so to a greater degree than rats receiving saline injections. These data indicate that interoceptive and psychosocial stressors support conditioned food avoidance and acceptance, respectively. Examination of c-Fos immunoreactivity revealed distinct neural activation by interoceptive and psychosocial stressors that could provide the neural basis underlying opposite direction of food acceptance learning.

A comparison between taste avoidance and conditioned disgust reactions induced by ethanol and lithium chloride in preweanling rats

Adult rats display taste avoidance and disgust reactions when stimulated with gustatory stimuli previously paired with aversive agents such as lithium chloride (LiCl). By the second postnatal week of life, preweanling rats also display specific behaviors in response to a tastant conditioned stimulus (CS) that predicts LiCl-induced malaise. The present study compared conditioned disgust reactions induced by LiCl or ethanol (EtOH) in preweanling rats. In Experiment 1 we determined doses of ethanol and LiCl that exert similar levels of conditioned taste avoidance. After having equated drug dosage in terms of conditioned taste avoidance, 13-day-old rats were given a single pairing of a novel taste (saccharin) and either LiCl or ethanol (2.5 g/kg; Experiment 2). Saccharin intake and emission of disgust reactions were assessed 24 and 48 hr after training. Pups given paired presentations of saccharin and the aversive agents (ethanol or LiCl) consumed less saccharin during the first testing day than controls. These pups also showed more aversive behavioral reactions to the gustatory CS than controls. Specifically, increased amounts of grooming, general activity, head shaking, and wall climbing as well as reduced mouthing were observed in response to the CS. Conditioned aversive reactions but not taste avoidance were still evident on the second testing day. In conclusion, a taste CS paired with postabsorptive effects of EtOH and LiCl elicited a similar pattern of conditioned rejection reactions in preweanling rats. These results suggest that similar mechanisms may be underlying CTAs induced by LiCl and a relatively high EtOH dose.

Latent inhibition of conditioned disgust reactions in rats

The present experiments, using the latent inhibition (LI) paradigm, evaluated the effect of nonreinforced exposure to saccharin on the acquisition of an LiCl-induced saccharin aversion as measured by conditioned disgust reactions in the taste reactivity test and conditioned taste avoidance in a consumption test. When rats were preexposed to saccharin by bottle exposure (Experiments 1 and 3), LI was evidenced only by conditioned taste avoidance (bottle testing), but not by conditioned disgust reactions (intraoral [IO] testing). On the other hand, when rats were preexposed to saccharin by IO infusion (Experiments 2 and 3), LI was evidenced only by conditioned disgust reactions, but not by conditioned taste avoidance. Experiment 4 showed that LI of conditioned disgust reactions does not appear to be affected by a context shift from preexposure to testing phases. These results show that the expression of LI of both conditioned taste avoidance and conditioned disgust reactions depends critically on a common method of flavor exposure during preexposure and testing.

Neuronal representation of conditioned taste in the basolateral amygdala of rats

Animals develop robust learning and long lasting taste aversion memory once they experience a new taste that is followed by visceral discomfort. A large body of literature has supported the hypothesis that basolateral amygdala (BLA) plays a critical role in the acquisition and extinction of such conditioned taste aversions (CTA). Despite the evidence that BLA is crucially engaged during CTA training, it is unclear how BLA neural activity represents the conditioned tastes. Here, we incorporated a modified behavioral paradigm suitable for single unit study, one which utilizes a sequence of pulsed saccharin and water infusion via intraoral cannulae. After conditioning, we investigated BLA unit activity while animals experience the conditioned taste (saccharin). Behavioral tests of taste reactivity confirmed that the utilized training procedure produced reliable acquisition and expression of the aversion throughout test sessions. When neural activity was compared between saccharin and water trials, half of the recorded BLA units (77/149) showed differential activity according to the types of solution. 76% of those cells (29/38) in the conditioned group showed suppressed activity, while only 44% of taste reactive cells (17/39) in controls showed suppressed activity during saccharin trials (relative to water trials). In addition, the overall excitability of BLA units was increased as shown by altered characteristics of burst activity after conditioning. The changes in BLA activity as a consequence of CTA were maintained throughout test sessions, consistent with the behavioral study. The current study suggests that the neuronal activity evoked by a sweet taste is altered as a consequence of CTA learning, and that the overall change might be related to the learning induced negative affect.

A learned flavor preference persists despite the extinction of conditioned hedonic reactions to the cue flavors

Learned flavor preferences can be strikingly persistent in the face of behavioral extinction. Harris, Shand, Carroll, and Westbrook (2004) suggested that this persistence may be due to flavor preference conditioning's producing a long-lasting change in the hedonic response to the conditioned stimulus (CS+) flavor. In the present study, the CS+ flavor was presented in simultaneous compound with 16% sucrose, whereas the CS- flavor was presented with 2% sucrose. During subsequent two- and one-bottle tests, the CS+ and CS- flavors were presented in 2% sucrose. Hedonic reactions during training and test were assessed using an analysis of the microstructure of licking behavior. Conditioning resulted in greater consumption of the CS+ than of the CS- that did not extinguish over repeated two- and one-bottle tests. The mean lick cluster size was higher for the CS+ than for the CS- only on the first cycle of tests. Since lick cluster size can be used as an index of stimulus palatability, the present results indicate that although the hedonic reaction to the CSs did change, this was not maintained across repeated tests. Thus, changes in the hedonic response to the conditioned flavors cannot explain the resistance to the extinction of learned flavor preferences.

Short Article: Microstructural Analysis of Ingestive Behaviour Reveals no Contribution of Palatability to the Incomplete Extinction of a Conditioned Taste Aversion

An analysis of the microstructure of licking responses was used to investigate the effects of conditioning and extinguishing a taste aversion. Rats received a single pairing of 8% fructose with lithium chloride (LiCl) while controls received unpaired exposure to fructose and LiCl. Pairing fructose with LiCl produced a reduction both in consumption and in the size of licking clusters. Subsequent exposure to fructose in the absence of LiCl produced some extinction of the taste aversion although at asymptote there was a residual difference in consumption between the taste aversion group and unpaired controls. In contrast the reduction in lick cluster size did completely extinguish. Previous analyses of licking microstructure indicate that lick cluster size is related to the palatability of the ingested solution. Thus these results indicate that although taste aversion learning initially reduces the palatability of the cue solution this reduction is not permanent. These results are discussed with reference to the possibility that preparatory behaviours are more resistant to extinction than are consummatory behaviours.

Amount of training and cue-evoked taste-reactivity responding in reinforcer devaluation

In two experiments, rats received minimal (16) pairings of one auditory conditioned stimulus (CS) cue with a sucrose reinforcer, and extensive (112) pairings of another auditory CS with that reinforcer. After sucrose was devalued by pairing it with lithium chloride in some rats (Devalue groups) but not others (Maintain groups), taste reactivity (TR) and other responses to unflavored water were assessed in the presence of the auditory CSs alone. The minimally trained CS controlled substantially more evaluative TR responses than the extensively trained CS. Those TR responses were hedonic (positive) in the Maintain groups, but aversive (negative) in the Devalue groups. By contrast, food cup entry and other responses thought not to reflect evaluative taste processing were controlled more by the extensively trained cue. These responses were reduced by sucrose devaluation comparably, regardless of the amount of training. The results suggest rapid changes in the content of learning as conditioning proceeds. Early in training, CSs may be capable of activating preevaluative processing of an absent food reinforcer that includes information about its palatability, but that capability is lost as training proceeds.

Effect of conditioning method and testing method on strength of lithium-induced taste aversion learning

Here the authors evaluated the effect of the method of conditioning (bottle or intraoral [IO] infusion) on the strength of a flavor-drug association when measured in a standard 1-bottle consumption test or when measured by IO infusion in a taste reactivity test. When tested with the bottle test in Experiment 1, rats conditioned by bottle displayed stronger taste avoidance than those conditioned by IO infusion. When tested for rejection reactions with the taste reactivity test in Experiment 2, rats conditioned by infusion displayed a stronger aversion than did rats conditioned by bottle. The results suggest that when the contextual cues of conditioning are similar at conditioning and testing, a stronger association is evident regardless of the individual specifics of each method. These results may shed light on recent reports that different neural mechanisms are involved in conditioning by active consumption and passive infusion.