Flavor Neophobia Research Articles

Increased basolateral amygdala metabolic activity during flavor familiarization: an experimental study

BackgroundNovel flavors elicit a cautious neophobic response which is attenuated as the flavor becomes familiar and safe. The attenuation of neophobia reveals the formation of a safe memory. Previous lesion studies in rats have reported that basolateral amygdala integrity is required for taste neophobia, but not neophobia to flavor, i.e., taste linked to an odorous component. Accordingly, immunohistochemical analyses show that novel tastes induced higher basolateral amygdala activity when compared to familiar ones. However, a different role of basolateral amygdala in flavor attenuation of neophobia is suggested by lesion studies using a vinegar solution. Studies assessing basolateral amygdala activity during flavor attenuation of neophobia are lacking. Thus, we quantified cytochrome oxidase as an index of basolateral amygdala activity along the first and second vinegar exposures in order to assess flavor neophobia and attenuation of neophobia.MethodsWe exposed adult male Wistar rats either once or twice to a 3% cider vinegar solution or water, and compared the basolateral amygdala, piriform cortex and caudate putamen brain metabolic activity using cytochrome c-oxidase histochemistry.ResultsWe found increased flavor intake and cytochrome c-oxidase histochemistry activity during the second exposure in basolateral amygdala, but not in the piriform cortex and caudate/putamen.ConclusionsThe main finding of the study is that BLA metabolic activity was higher in the group exposed to a familiar vinegar solution than in the groups exposed to either water or a novel vinegar solution.

Differential activity pattern of c-Fos in the nucleus accumbens between adult and aged rats during flavor recognition memory

Previous studies have addressed the role of the nucleus accumbens core (NAcbC) and shell (NAcbSh) in taste aversion learning and in the processing of taste palatability which is affected by aging. However, little is known about its implication in safe taste memory and the aging impact. To explore the role of the NAcb in flavor neophobia and its attenuation during aging, we applied c-Fos immunohistochemistry as an index of neural activity of the NAcbC and NAcbSh. Twenty one adult (5-month-old) and 24 aged (24-month-old) male Wistar rats were exposed to a 3% cider vinegar solution for 1, 2 or 6 consecutive days (n = 7 adult and n = 8 aged rats per group). Aged rats exhibited slower attenuation of flavor neophobia than adult rats. Adult rats showed increased NAcbSh c-Fos activity on day 2 compared to days 1 and 6, while this increase was delayed to day 6 in aged rats. There were no differences in the number of NAcbC c-Fos positive cells. This suggests that changes in the activity of neural circuits of palatability processing during normal aging could contribute to the slower attenuation of flavor neophobia in aged rats.

Flavor recognition memory related activity of the posterior piriform cortex in adult and aged rats

The relationship between the piriform cortex and flavor recognition memory was investigated in adult and aged rats. By using c-Fos immunohistochemistry, we assessed the piriform cortex activity induced by flavor familiarity. The results indicated increased activity in the rostral region of the posterior piriform cortex elicited by the most familiar cider vinegar solution after six exposures. Aged rats exhibited overall increased activity in the posterior, but not the anterior piriform cortex, which was not related to flavor familiarity. This suggests that the posterior piriform cortex is related to flavor recognition memory and that aging modifies its activity pattern which might underlie their slower attenuation of flavor neophobia.

Flavor and object recognition memory impairment induced by excitotoxic lesions of the perirhinal cortex

Recognition memory is based on the ability to assess the familiarity of a previously encountered stimulus. It can be approached using tests for different sensorial modalities. Excitotoxic lesions of the perirhinal cortex (Prh) were performed in order to assess the relevance of its integrity for object and flavor recognition memory. Object recognition memory was impaired with a 24h retention interval. Flavor neophobia attenuation was prevented on a second encounter with the tastant. These results support a role of the perirhinal cortex in mediating the transition from novel to familiar, both in object and flavor recognition memory.

Contextual control of flavor neophobia

The role of context in the retrieval of learned information has been widely analyzed in the associative learning domain. However, evidence about the effect of context on flavor memory retrieval is more limited. We have carried out four experiments with rats testing for possible interactions between neophobia habituation and the context in which flavors are presented, by manipulating prior experience with contexts. Our results point to the relevance of context familiarity for the establishment and recovery of a safe taste memory trace. More specifically, the use of the animals' home cages as experimental context favored neophobia habituation (Experiments 1A and 2), reduced dopamine levels induced by administration of the dopamine D1-like receptor antagonist SCH-23390 disrupted neophobia habituation when tested in presence of a new context (Experiment 1B), and testing in the animal's home cage increases the amount of flavor consumed, even when such flavor had a previous history of aversive conditioning (Experiment 3). We propose that exploring context without aversive consequences generates a safe memory trace of such context that becomes in the basis of increased flavor consumption.

Dexamethasone and flavor neophobia in hamsters

The effects of dexamethasone (DEX) on the reactions of male and female hamsters to novel flavors were studied in an attempt to assess the role of novelty-induced stress in flavor neophobia. DEX is a synthetic glucocorticoid of wide-ranging effects including the blocking of adrenal response to stress in pregnant hamsters. If reactions to novel flavors are mediated by stress, DEX-treated hamsters should show an attenuation of flavor neophobia in relation to nontreated animals. Neophobic reactions were assessed by comparing the hamsters’ latency to pouch a novel-flavored nut in comparison with a familiar unflavored one. Flavor neophobia was manifested in response to novel salty, sweet, or sour nuts when male and female hamsters were tested under the control (no-DEX) condition. Similar results were obtained with DEX-treated animals, with one exception. There was an interaction indicating that DEX-treated females showed enhanced neophobic reactions to a sucrose-flavored nut. The theoretical implications of these findings are discussed.

Age dependency in neophobia: Its influence on taste-aversion learning and the flavor-preexposure effect in rats

Five experiments were conducted to evaluate various aspects of stimulus preexposure effects on conditioned saccharin aversion in rats of three age groups: weanling (19–25 days), young-adult (92–170 days), and old-age (680–850 days). In Experiment 1, flavor neophobia was examined. Only the young-adult and old-age animals showed evidence of neophobia. Furthermore, habituation of the neophobic reaction differed for these two age groups. Using a brief to moderately long flavor-preexposure period and an intense US (Experiment 2), we demonstrated that the youngest age group was most likely to exhibit retarded conditioning as a result of preexposure to the flavor CS. Using a weaker US and a moderately long saccharin-preexposure period (Experiment 3), age differences in conditioning resulting from preexposure to the flavor CS were reduced. When animals were preexposed to saccharin continuously for 48 h (Experiment 4), age differences in the preexposure effect were not evident. In Experiment 5, the intensity of the US was reduced to determine whether floor effects in the previous experiments had masked age differences in the ability of nonpreexposed rats to acquire an aversion to saccharin. Results indicated that taste-aversion learning was directly related to age. Although open to other interpretations, the results support the notion that the flavor-preexposure effect is influenced by the initial level of flavor neophobia. They also suggest that systematic parametric variation is sometimes necessary to obtain an accurate description of age differences in learning.

Developmental flavor experience affects utilization of odor, not taste in toxiphobic conditioning

Feeding experiences were varied in developing rats and the effects upon flavor neophobia and lithium chloride-induced flavor aversions were observed. In Experiment 1, nursing experience of neonate rats was reduced by artificial feeding via intragastric cannula; the rats then were tested with apple juice paired with lithium chloride injection at weaning or maturity. Conditioned aversions were not affected, but neophobia to novel apple juice was attenuated in artificially-reared rats tested at maturity. In Experiment 2, rats received enriched feeding experience after weaning, which consisted of (a) obtaining many complex flavors, a few of which were paired with poisoning, effortlessly in the home cage, or (b) foraging for various foods on an elevated maze. No dramatic effects on neophobia or conditioned taste aversion for saccharin water were apparent. In Experiment 3, rats were given experience after weaning with vanilla-scented water either paired or unpaired with quinine water, and then tested with the odor of almond or that odor compounded with saccharin water for neophobia and lithium-induced aversions. Flavor-experienced rats exhibited more pronounced odor conditioning and more resistance to extinction of the odor aversion after both simple and compound conditioning. In contrast, saccharin taste aversions were relatively unchanged. Apparently, enriched feeding and drinking experience facilitates the utilization of odor more than taste cues.

Persistence of flavor neophobia as an indicator of state-dependent retention induced by pentobarbital, stress, and estrus

As fear conditioning paradigms have failed to find state-dependent retention (SDR) in rats dissociated by the estrous state, an attempt was made to establish a paradigm involving the attenuation of flavor neophobia as a more sensitive indicator of SDR. Experiment 1 established that the paradigm did indicate SDR in male rats dissociated by pentobarbital (10 mg/kg). Experiment 2 generalized the utility of the technique by employing an endogenous state, shock-induced stress, as the dissociative state. Having proved to be an indicator of SDR in the first two experiments, the technique was used in Experiment 3 to investigate the possible dissociative characteristics of the estrous state in female rats. Estrus was investigated because, due to the hormonal condition during estrus, it may provide a useful tool in investigating the dissociative aspects of human perimenstrual periodic psychosis. In Experiment 3, using ovariectomized female rats brought into estrus by injection of estrogen and progesterone in a sequence and amount comparable to the natural cycle, estrus was not demonstrated to be dissociative, i.e., did not produce SDR. The implications of these results in relation to perimenstrual periodic psychosis were discussed.

Flavor—illness aversions: Potentiation of odor by taste is disrupted by application of novocaine into amygdala

Taste and odor have different properties in toxiphobic conditioning. When each is used alone, taste becomes aversive when followed by immediate or delayed poison, while odor becomes aversive only if followed by immediate poison. However, if odor and taste are presented as a compound and followed by delayed poison, then odor does become aversive when tested alone. It is as if taste has potentiated the odor signal. Several experiments assessed the role of the amygdala in this potentiation effect by anesthetizing the amygdala with 10% novocaine. Novocaine applied 30 min before presentation (Pre-CS) of an odor-taste compound disrupted the potentiated odor aversion but not the taste aversion. In contrast, novocaine applied 1 min after the compound odor-taste or 1 min prior to LiCl poison did not dissociate odor and taste aversions; both odor and taste aversions were facilitated. Novocaine applied 30 min before an odor alone also disrupted an odor aversion induced by immediate LiCl. But identical treatment did not disrupt odor avoidance conditioned by immediate foot-shock, suggesting that amygdala anesthesia does not simply produce anosmia. Pre-CS novocaine treatment also disrupted flavor neophobia prior to conditioning. The results suggest that novocaine applied to the amygdala disrupts the integration of odor with taste and illness during toxiphobic conditioning.

The determinants of exploration and neophobia

The literature dealing with the factors that determine approach and avoidance responses to novelty is reviewed and evaluated and suggestions for further clarifying research are incorporated. Only those studies that measure exploration in a choice-type task are included because of the interpretive difficulties associated with forced-exploration tasks (e.g., the open-field). The determinants that are reviewed include changes in stimulus novelty and changes in responsiveness independent of stimulus novelty. The latter factors include physiological variables such as lesions and drugs. The juxtaposition of some recent data with older findings has helped to reveal similarities that may reflect the presence of similar underlying mechanisms. The status of the concepts of optimal arousal level and adaptation level is briefly discussed and it is concluded that they are incapable of encompassing the range of data available. In addition, some methodological issues are considered and it is concluded that concentration on the results of choice-type tasks has helped resolve some of the inconsistencies and ambiguities that had arisen through the use of forced-exploration tasks. The necessary requirements for the demonstration of flavor neophobia are also discussed.

Enhancing the expression of flavor neophobia: Some effects of the ingestion-illness contingency.

Enhancing the expression of flavor neophobia: Some effects of the ingestion-illness contingency.

Enhancing the expression of flavor neophobia: some effects of the ingestion-illness contingency.

Four experiments investigating factors contributing to enhanced ingestional neophobia are reported. Rats administered lithium chloride following ingestion of a novel coffee solution showed an enhanced neophobia reaction to vinegar and casein. This enhancement was specific to the novelty of both the conditioning and test fluids and was not observed in animals receiving noncontingent toxicosis. Poisoning alone, however, mediated a nonspecific fluid suppression that persisted for approximately two drinking sessions following treatment. In contrast to other experiments, the operation of generalization was detected only when a novel flavor was the test fluid, suggesting that neophobia enhancement is at least partially mediated by a conditioned novelty aversion resulting from the novel flavor-lithium contingency.

Reinforcing properties of novel and familiar solutions of saccharin for rats

To evaluate the influence of flavor neophobia on the reinforcing properties of edibles, the leverpress behavior of rats was reinforced with either a novel or a familiar 2.0% solution of sodium saccharin following preliminary training with water reinforcement. The familiar saccharin flavor was much more effective in reinforcing instrumental behavior than was the novel saccharin. In fact, reinforcement with a novel saccharin flavor initially did not stimulate more responding than what was observed in another group of subjects given no reward. Repeated training with the novel saccharin reward gradually increased the rate of leverpressing, consistent with a counterconditioning interpretation. However, this counterconditioning did not produce the increased resistance to extinction in the final phase of the experiment which is predicted by the general theory of persistence (Amsel, 1972).