Effect of previous taste experiences on taste neophobia in young-adult and aged rats

Systematic review of pharmacological treatments that reduce conditioned taste aversions in rodents: A potential animal model of pediatric feeding disorder and avoidant/restrictive food intake disorder (ARFID)

Avoidant/restrictive food intake disorder (ARFID) is diagnosed when food avoidance leads to clinically significant nutritional, weight/growth, or psychosocial impairment. As many as 81.5% of children and adolescents diagnosed with ARFID have a history of a medical condition associated with pain, fatigue, or malaise. ARFID is diagnosed and treatment begins after the medical condition is resolved but food avoidance remains. Effective treatment involves repeated exposure to eating food and related stimuli aimed at creating inhibitory learning to counteract learned fears and aversions. Treatment usually involves positive reinforcement of food approach behavior and escape extinction/response prevention to eliminate food avoidant behavior. To shed light on the neural mechanisms that may maintain ARFID and to identify candidate pharmacological treatments for adjuncts to behavioral interventions, this paper systematically reviews research on drug treatments that successfully reduce conditioned taste aversions (CTA) in animal models by disrupting reconsolidation or promoting extinction. The mechanism of action of these treatments, brain areas involved, and whether these CTA findings have been used to understand human eating behavior are assessed. Collectively, the results provide insight into possible neural mechanisms associated with resuming oral intake following CTA akin to the therapeutic goals of ARFID treatment and suggest that CTA animal models hold promise to facilitate the development of interventions to prevent feeding problems. The findings also reveal the need to investigate CTA reduction in juvenile and female animals and show that CTA is rarely studied to understand disordered human feeding even though CTA has been observed in humans and parallels many of the characteristics of rodent CTA.

Aging in nucleus accumbens and its impact on alcohol use disorders

Ethanol is one of the most widely consumed drugs in the world and prolonged excessive ethanol intake might lead to alcohol use disorders (AUDs), which are characterized by neuroadaptations in different brain regions, such as in the reward circuitry. In addition, the global population is aging, and it appears that they are increasing their ethanol consumption. Although research involving the effects of alcohol in aging subjects is limited, differential effects have been described. For example, studies in human subjects show that older adults perform worse in tests assessing working memory, attention, and cognition as compared to younger adults. Interestingly, in the field of the neurobiological basis of ethanol actions, there is a significant dichotomy between what we know about the effects of ethanol on neurochemical targets in young animals and how it might affect them in the aging brain. To be able to understand the distinct effects of ethanol in the aging brain, the following questions need to be answered: (1) How does physiological aging impact the function of an ethanol-relevant region (e.g., the nucleus accumbens)? and (2) How does ethanol affect these neurobiological systems in the aged brain? This review discusses the available data to try to understand how aging affects the nucleus accumbens (nAc) and its neurochemical response to alcohol. The data show that there is little information on the effects of ethanol in aged mice and rats, and that many studies had considered 2-3-month-old mice as adults, which needs to be reconsidered since more recent literature defines 6 months as young adults and >18 months as an older mouse. Considering the actual relevance of an aged worldwide population and that this segment is drinking more frequently, it appears at least reasonable to explore how ethanol affects the brain in adult and aged models.

Insula to mPFC reciprocal connectivity differentially underlies novel taste neophobic response and learning in mice

To survive in an ever-changing environment, animals must detect and learn salient information. The anterior insular cortex (aIC) and medial prefrontal cortex (mPFC) are heavily implicated in salience and novelty processing, and specifically, the processing of taste sensory information. Here, we examined the role of aIC-mPFC reciprocal connectivity in novel taste neophobia and memory formation, in mice. Using pERK and neuronal intrinsic properties as markers for neuronal activation, and retrograde AAV (rAAV) constructs for connectivity, we demonstrate a correlation between aIC-mPFC activity and novel taste experience. Furthermore, by expressing inhibitory chemogenetic receptors in these projections, we show that aIC-to-mPFC activity is necessary for both taste neophobia and its attenuation. However, activity within mPFC-to-aIC projections is essential only for the neophobic reaction but not for the learning process. These results provide an insight into the cortical circuitry needed to detect, react to- and learn salient stimuli, a process critically involved in psychiatric disorders.

Prefrontal cortex activity patterns during taste neophobia habituation in adult and aged rats

Age-related memory decline has been associated with changes in the medial prefrontal cortex (mPFC) function. In order to explore the role of mPFC in taste recognition memory, we have assessed mPFC c-Fos immunoreactivity in adult (5-month-old) and aged (24-month-old) male Wistar rats during the first (Novel), second (Familiar I), and sixth (Familiar II) exposure to a cider vinegar solution. Adult brains showed higher c-Fos expression in the ventral but not the dorsal region of mPFC during the second taste exposure. Interestingly, old brains exhibited an altered activity pattern selectively in the dorsal peduncular cortex (DP) which can be associated with a delayed attenuation of vinegar neophobia in this group. These results support the involvement of this area in the formation of safe taste memory. Further research is needed for understanding the role of DP in taste recognition memory and the impact of aging on it.

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.

The role of the gustatory cortex in incidental experience-evoked enhancement of later taste learning

The strength of learned associations between pairs of stimuli is affected by multiple factors, the most extensively studied of which is prior experience with the stimuli themselves. In contrast, little data is available regarding how experience with "incidental" stimuli (independent of any conditioning situation) impacts later learning. This lack of research is striking given the importance of incidental experience to survival. We have recently begun to fill this void using conditioned taste aversion (CTA), wherein an animal learns to avoid a taste that has been associated with malaise. We previously demonstrated that incidental exposure to salty and sour tastes (taste preexposure-TPE) enhances aversions learned later to sucrose. Here, we investigate the neurobiology underlying this phenomenon. First, we use immediate early gene (c-Fos) expression to identify gustatory cortex (GC) as a site at which TPE specifically increases the neural activation caused by taste-malaise pairing (i.e., TPE did not change c-Fos induced by either stimulus in isolation). Next, we use site-specific infection with the optical silencer Archaerhodopsin-T to show that GC inactivation during TPE inhibits the expected enhancements of both learning and CTA-related c-Fos expression, a full day later. Thus, we conclude that GC is almost certainly a vital part of the circuit that integrates incidental experience into later associative learning.

Food neophobia in wild and laboratory rats (multi-strain comparison)

Although empirical studies comparing neophobia in wild and laboratory rats have been conducted in the past, a few decades have passed since most of them were completed. This is a substantial period of time in the case of fast-breeding animals such as rats. Equally important are the inconsistencies in research findings with respect to comparisons between wild and laboratory rats, and within domesticated strains. As well as having the aim of updating knowledge of neophobia among different types of rats, the present experiment was also an attempt to isolate a specific fear of a new food from a general fear of a novel object. The procedure was that rats accustomed to one type of food served in a specific location and in a familiar container were given a different type of food. Test trials were preceded by food deprivation. The following variables were measured: feeding latency, the pace of eating, the number of approaches to the container, and the number of times food was sampled in each trial. The amount of food consumed in each trial was weighed and also taken into account. Grooming time served as the measure of stress among the rats in the experiment. The results of the experiment did not confirm the assertion of some authors that wild rats avoid eating unfamiliar foods. All groups demonstrated only a temporary decrease in the amount of food consumed, the magnitude of which was similar in all strains. No evidence of particularly low neophobia in albino rats was found. However, the behavioral symptoms indicated higher levels of stress in wild rats compared to the other groups.

Latent inhibition and facilitation of conditioned taste aversion in preweanling rats

Early in ontogeny, taste preexposure has been found to induce latent inhibition as well as produce a facilitation of conditioned taste aversion (CTA). In this study, the effect of taste preexposure on CTA was investigated in 13-14 day old rats as a function of taste preexposure (0, 1, or 3 trials) and unconditioned stimulus intensity (LiCl: 0, 0.15, or 0.30 M). After one conditioning trial, with the low intensity US, an aversion was only observed after taste preexposure (facilitation). When using the strong US, an aversion was found without preexposure while latent inhibition was observed with 3 preexposure trials. In conclusion, stimulus preexposure can either facilitate conditioning or produce latent inhibition in infant rats, depending on the amount of stimulus preexposure and the intensity of the US.

Licking microstructure reveals rapid attenuation of neophobia

Many animals hesitate when initially consuming a novel food and increase their consumption of that food between the first and second sessions of access-a process termed attenuation of neophobia (AN). AN has received attention as a model of learning and memory; it has been suggested that plasticity resulting from an association of the novel tastant with "safe outcome" results in a change in the neural response to the tastant during the second session, such that consumption increases. Most studies have reported that AN emerges only an hour or more after the end of the first exposure to the tastant, consistent with what is known of learning-related plasticity. But these studies have typically measured consumption, rather than real-time behavior, and thus the possibility exists that a more rapidly developing AN remains to be discovered. Here, we tested this possibility, examining both consumption and individual lick times in a novel variant of a brief-access task (BAT). When quantified in terms of consumption, data from the BAT accorded well with the results of a classic one-bottle task-both revealed neophobia/AN specific to higher concentrations (for instance, 28mM) of saccharin. An analysis of licking microstructure, however, additionally revealed a real-time correlate of neophobia-an explicit tendency, similarly specific for 28-mM saccharin, to cut short the initial bout of licks in a single trial (compared with water). This relative hesitancy (i.e., the shortness of the first lick bout to 28-mM saccharin compared with water) that constitutes neophobia not only disappeared between sessions but also gradually declined in magnitude across session 1. These data demonstrate that the BAT accurately measures AN, and that aspects of AN-and the processes underlying familiarization-begin within minutes of the very first taste.

Role of the insular cortex in taste familiarity

Determining the role of the main gustatory cortical area within the insular cortex (IC), in conditioned taste aversion (CTA) has been elusive due to effective compensatory mechanisms that allow animals to learn in spite of lacking IC. IC lesions performed before CTA training induces mild if any memory impairments, while IC lesions done weeks after CTA produce amnesia. IC lesions before taste presentation have also been shown not to affect taste familiarity learning (attenuation of neophobia). This lack of effect could be either explained by compensation from other brain areas or by a lack of involvement of the IC in taste familiarity. To assess this issue, rats were bilaterally IC lesioned with ibotenic acid (200-300 nl.; 15 mg/ml) one week before or after taste familiarity, using either a preferred (0.1%) or a non-preferred (0.5%) saccharin solution. Rats lesioned before familiarity showed a decrease in neophobia to both solutions but no difference in their familiarity curve or their slope. When animals were familiarized and then IC lesioned, both IC lesioned groups treated the solutions as familiar, showing no differences from sham animals in their retention of familiarity. However, both lesioned groups showed increased latent inhibition (or impaired CTA) when CTA trained after repeated pre-exposures. The role of the IC in familiarity was also assessed using temporary inactivation of the IC, using bilateral micro-infusions of sodium channel blocker bupivacaine before each of 3 saccharin daily presentations. Intra-insular bupivacaine had no effects on familiarity acquisition, but did impair CTA learning in a different group of rats micro-infused before saccharin presentation in a CTA training protocol. Our data indicate that the IC is not essentially involved in acquisition or retention of taste familiarity, suggesting regional dissociation of areas involved in CTA and taste familiarity.

Effect of chronic hCG administration on Alzheimer's-related cognition and A beta accumulation in PS1KI mice

Age-associated changes in the reproductive hormones-the gonadal steroid hormones and the gonadotropins-have been identified as potential risk factors for Alzheimer's disease (AD). However, levels of gonadotropins and estrogens are closely linked in vivo, and it has proven difficult to separate the effects of gonadotropins from the well-documented estrogenic effects on AD-related neuropathology in experimental models of menopause. To assess the effects of gonadotropins on cognition and AD biochemical markers independent of estrogenic effects, a potent analog of luteinizing hormone [human chorionic gonadotropin (hCG)] was administered to ovariectomized presenilin1 knock-in mice (PS1KI). Gonadotropin administration was found to induce hyperactivity and anxiety (Open Field Maze and Taste Neophobia Task) and working memory dysfunction, without altering reference memory (Morris Water Maze). Although gonadotropin administration modestly altered β amyloid (Aβ40) levels, levels of the longer more toxic form (Aβ42) were unaffected. Furthermore, altered Aβ40 levels were not associated with observed behavioral and cognitive impairments. These findings provide proof, in principle, that the gonadotropin hormones play a role in the modulation of AD-related behavior, cognition, and neuropathology.

Increased liking for a solution is not necessary for the attenuation of neophobia in rats

Recent evidence suggests that liking and wanting of food rewards can be experimentally dissociated (e.g., Berridge, 1996); this dissociation extends to attenuated neophobia in the present study. Rats tend to eat less of a novel food than a familiar food, a phenomenon called neophobia. The present experiments evaluated whether attenuation of neophobia by prior exposure reflects enhanced liking of the flavor using the Taste Reactivity (TR) test. In Experiment 1, rats given five 10-s TR trials with water or various concentrations of saccharin solution (0.1%, 0.2%, 0.5%) did not show a change in the number of hedonic reactions displayed across trials. However, in a subsequent consumption test from a bottle containing 0.25% saccharin solution, rats with no prior saccharin exposure (group water) consumed less than rats with prior saccharin exposure; that is they displayed neophobia. In Experiment 2, whether rats received five 10-s TR trials with water or 0.5% saccharin solution, they did not display a difference in hedonic reactions to 0.25% saccharin solution in two 5-min TR test trials. These results suggest that the attenuation of neophobia is evidenced as an increase in the tendency to approach a bottle containing the flavored solution (wanting), but not as an enhanced liking of that solution.

The effect of taste familiarity on intake and taste reactivity in infant rats

With infant rats, unlike with adults, increased intake of a taste after mere exposure to this stimulus is not consistently found; this has sometimes been interpreted as a failure by the immature subject to recognize tastes as familiar. We studied the effect of preexposure to a tastant, measuring taste reactivity and intake in 14-day-old rats. Familiarity increased hedonic response to sucrose, but also increased aversive response to quinine and ethanol. With the sucrose-quinine compound, familiarity increased both the hedonic and the aversive reaction to the stimulus. In no case was a differential reactivity to water observed. Significant increased intake after familiarization was only found with quinine or the sucrose-quinine compound. Results indicate that in infant rats, and with the present parameters, taste familiarity enhances responsiveness to these stimuli, an effect not always accompanied by detectable changes in intake.

Peculiar modulation of taste aversion learning by the time of day in developing rats

The ontogeny of the temporal context modulation of conditioned taste aversion was studied in male Wistar rats using a palatable 1% NaCl solution. A procedure that included two saline preexposures, a single pairing saline-lithium chloride (0.15 M; 1% b.w.) either at the same or a different time of day of preexposures and a one-bottle test at the same time than preexposure was applied. Four age groups (PN32, PN48, PN64, and PN100) covering the complete range from adolescence to the adult period were tested. The results showed no effect of a temporal context shift in PN32. A peculiar enhancement of temporal context-specific saline aversions was exhibited by PN48 and PN64 rats, while the adult typical temporal context specificity of latent inhibition was only evident in PN100 rats. The results are discussed in terms of the peculiar brain functional organization during a protracted adolescence period.