Effects of CS concentration in taste-aversion learning: Problems in assessing aversion strength

The concentration of saccharin (CS) solutions was varied between groups of rats in four taste-aversion learning experiments, using lithium chloride as the aversive agent (US). Saccharin intake was measured on four one-bottle presentations yielding data on initial taste neophobia as well as postconditioning aversion strength. The results indicated that saccharin intake declined progressively with increasing concentrations on the first presentation, and that preconditioning intake to a large extent predicted postconditioning intake. In an attempt to correct postconditioning values for differences of saccharin intake on the preceding trial, a taste suppression ratio (TSR) was computed as the quotient of the amounts of saccharin consumed by individual rats on consecutive presentations. In general, the TSR was found to be too insensitive to detect differences of conditioning strength acquired by different saccharin concentrations. Alternative approaches to the CS-concen-tration problem in taste-aversion learning are discussed.     

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.     

Potentiation of latent inhibition

Rats were given exposure either to an odor (almond) or a compound of odor plus taste (almond plus saline), prior to training in which the odor served as the conditioned stimulus. It was found, for both appetitive and aversive procedures, that conditioning was retarded by preexposure (a latent inhibition effect), and the extent of the retardation was greater in rats preexposed to the compound (i.e., latent inhibition to the odor was potentiated by the presence of the taste). In contrast, the presence of the taste during conditioning itself overshadowed learning about the odor. We argue that the presence of the salient taste in compound with the odor enhances the rate of associative learning, producing a rapid loss in the associability of the odor. This loss of associability will generate both overshadowing and the potentiation of latent inhibition that is observed after preexposure to the compound.     

Gastrointestinal reactivity in rats lacking anterior insular neocortex

Behavioral and physiological studies have shown that the insular neocortex participates in a wide variety of special visceral processes, in particular, the higher-order integration of taste stimuli and the regulation of autonomic activity. The present experiment examined the involvement of the anterior insular (gustatory) neocortex (AIGN) in gastrointestinal reactivity and taste learning in rats by the use of a modified taste aversion training procedure. Normal rats and rats lacking AIGN demonstrated similar "illness thresholds" to the early onset symptoms of ingested LiCl. Conversely, animals lacking AIGN showed significant impairments in taste aversion learning ability. From a consideration of several research findings it is concluded that animals lacking AIGN can normally perceive tastes and illness, but these animals experience reliable impairments in taste aversion learning ability.     

Conditioned changes in pain reactivity I: A discrete CS elicits hypoalgesia, not hyperalgesia, on the tail-flick test

Prior work suggests that a stimulus which has been paired with an aversive event may elicit either an increase or decrease in pain reactivity. In four experiments we attempted to isolate the variables which determine the direction of the conditioned response. In all experiments, one stimulus was paired with shock (the CS+) while another was nonreinforced (the CS−). Pain reactivity was assessed by measuring tail withdrawal from radiant heat (the tail-flick test). Experiment 1 tested whether the amount of training plays a critical role. We found that the CS+ elicits longer tail-flick latencies irrespective of whether subjects received 6 or 24 CS-US pairings. Although there was some evidence that subjects may exhibit a general hyperalgesia at 2 min after the presentation of the conditioned stimulus, the results of Experiment 2 revealed that this effect was an artifact caused by differences in the amount of time between test trials. The results of Experiment 2 also showed that the difference observed between the CS+ and CS− was not due to a CS− induced hyperalgesia. Rather, it reflects a CS+ induced hypoalgesia. Experiment 3 evaluated whether the time of day at which testing occurs is important. We found that subjects exhibit conditioned hypoalgesia when tested in either the light or dark portion of the light-dark cycle. In Experiment 4 we examined whether it matters if subjects are tested in the training context or a neutral context. We found that subjects exhibit conditioned hypoalgesia during the CS+ irrespective of where they are tested. The results suggest that a CS+ elicits hypoalgesia, not hyperalgesia, on the tail-flick test over a wide range of conditions.     

Aversiveness of denatonium saccharide and quinine in grasshopper mice Onychomys leukogaster

The responses toward denatonium saccharide and quinine by three rodent species differ from those exhibited by humans. This study measured the relative aversiveness of these two compounds in grasshopper mice (Onychomys leucogaster) to determine if this species responded similarly to or differently from these other rodents. In a choice test, the mice clearly preferred quinine over denatonium saccharide at concentrations of 1/10,000 and 1/50,000, but only marginally preferred quinine at a concentration of 1/1000. When denatonium or quinine was paired with toxicosis, both suppressed drinking at concentrations from 1/10,000 to 1/50,000, but only denatonium suppressed drinking at 1/100,000. Neither compound suppressed drinking at 1 part in million. Grasshopper mice perceived denatonium saccharide as more aversive than quinine in both experiments. Such a response resembled the behavior of humans more than that of the other three rodents previously tested.     

Differential effects of beta-adrenergic receptor blockade in basolateral amygdala or insular cortex on incidental and associative taste learning

The importance of central β-adrenergic system has been essentially investigated in aversive/emotional learning tasks. However, recent data suggest that the β-adrenergic system is also required for incidental taste learning. In the present study we evaluated in rats whether β-adrenergic receptor activity is required for taste habituation, an incidental taste learning, and also for conditioned taste aversion (CTA) learning, an associative learning. To address this issue, a low dose of the β-adrenergic antagonist propranolol was infused before learning in either the basolateral amygdala (BLA) or the insular cortex (IC), two forebrain areas reported to play a key role in taste memory formation. Incidental taste learning was assessed using a single presentation of the sweet taste saccharin 0.1%, which is sufficient to increase saccharin consumption (relative to water baseline) during a second presentation. CTA was assessed by pairing the first saccharin 0.1% presentation with a delayed gastric malaise, thus causing a decrease in saccharin consumption (relative to water baseline) during a second presentation. Propranolol infusion in BLA (1 μg/0.2μl) or IC (2.5 μg/0.5 μl) before the first taste exposure impaired incidental taste learning but did not affect CTA. These results highlight the important role played by the β-adrenergic receptor activation in cortical and amygdaloid structures during taste learning. Moreover, they are the first to suggest that incidental learning is more sensitive to blockade of noradrenergic system than associative learning.     

Failure of serial taste-taste compound presentations to produce overshadowing of extinction of conditioned taste aversion

Two experiments were conducted to study overshadowing of extinction in a conditioned taste aversion preparation. In both experiments, aversive conditioning with sucrose was followed by extinction treatment with either sucrose alone or in compound with another taste, citric acid. Experiment 1 employed a simultaneous compound extinction treatment and found results indicative of overshadowing of extinction. By contrast, Experiment 2, in which extinction treatment involved serial compound presentations, failed to obtain evidence of overshadowing of extinction. The results of Experiment 2 indicate that the serial presentation of two tastes was processed as equivalent to the separate presentation of the tastes. The results are discussed in relation to: (1) Convergent evidence from research on latent inhibition, (2) competing theories of learning and, (3) their possible adaptive value in food-selection learning.     

Conditioned taste aversion and Ca/calmodulin-dependent kinase II in the parabrachial nucleus of rats

Bielavska and colleagues (Bielavska, Sacchetti, Baldi, & Tassoni, 1999) have recently shown that KN-62, an inhibitor of calcium/calmodulin-dependent kinase II (CaCMK), induces conditioned taste aversion (CTA) when introduced into the parabrachial nucleus (PBN) of rats. The aim of the present report was to assess whether activity of CaCMK in the PBN is changed during CTA. We induced CTA in one group of rats by pairing saccharin consumption with an ip injection of lithium chloride. Another group of rats received lithium alone (without being paired with saccharin consumption) to test whether lithium has an effect on CaCMK in the PBN, independent of those effects due to training. In animals receiving CTA training, CaCMK activity in extracts of PBN was reduced by approximately 30% at the postacquisition intervals of 12, 24, and 48 h, compared to control animals receiving saccharin with saline injection. By 120 h after CTA training, no effect on CaCMK was present. At those postacquisition intervals showing CaCMK activity effects due to CTA, there were no effects attributable to lithium alone. Lithium alone produced only a short-lasting reduction in CaCMK activity (at 20 min a 30% decrease, at 60 min a 23% decrease; and at 6, 12, and 24 h no decrease). The time course of lithium-induced effects differed markedly from that of CTA training. All changes were Ca2+/- -dependent; we did not observe any changes in Ca-independent activity. CTA effects on CaCMK were selective for PBN, insofar as we did not observe any CTA effects on CaCMK in the visual cortex, a brain region unrelated to taste pathways. Since CTA produces a relatively long-lasting reduction in CaCMK activity (lasting 2 days or more) specifically in the PBN, which is critical a relay for taste information, the reduction of CaCMK activity may enable the consolidation of taste memory in an aversive situation.     

Odor as a conditioned inhibitor: Applicability of the Rescorla-Wagner model to feeding behavior

Hooded rats were given conditioned inhibition training in which the taste of saccharin alone was always followed by induced illness, but the taste of saccharin plus the odor of amyl acetate was not. In a series of three subsequent tests—summation, enhancement of conditioning, and retardation—it was demonstrated that the odor had acquired active inhibitory properties. The results paralleled those obtained with more traditionally studied stimuli and techniques and hence were found to be readily predictable from a recent model of conditioning set forth by Rescorla and Wagner (1972).     

Context-dependent latent inhibition in taste aversion learning

The effects of taste stimulus preexposure, either in the presence or in the absence of a specific contextual cue consisting of a specific noise-producing bottle, upon the conditioning and testing of conditioned taste aversions to the taste (saccharin) plus context (noisy-bottle) compound stimulus were investigated. Four groups of rats were given preexposure trials (latent inhibition) to either: (1) novel saccharin in novel noisy bottles, (2) novel saccharin in familiar silent bottles, (3) familiar water in novel noisy bottles, (4) familiar water in familiar silent bottles, in six trials. During conditioning, saccharin was presented in the noisy bottles followed by lithium chloride for all the groups. At testing, saccharin was presented in the noisy bottles for both one-bottle and two-bottle tests of aversion. It was indicated that the conditioning decrement produced after both saccharin and noisy bottle preexposure was overwhelmingly greater than any produced after preexposure to the elements. These results, discussed in relation to current theories of latent inhibition and perceptual learning, further underline the overwhelming significance of exteroceptive contextual elements in conditioned taste aversions.     

The addition of saccharin to taste cues affects taste preference conditioning in thirsty rats

Previous failures to condition preferences for the unacceptable taste cues sucrose octaacetate (SOA) and citric acid (CA) using a reverse-order, differential conditioning procedure (Forestell & LoLordo, 2000) may have been the result of low consumption of the taste cues in training or of their relatively low acceptability to rats that are thirsty and hungry. In the present study, rats that were thirsty but not hungry readily consumed the SOA and CA conditioned stimulus solutions in training. In test, stronger preferences were displayed for CS+ if the taste cues had been mixed in water (i.e., for the previously unacceptable taste cues) than if they had been made more acceptable by the addition of saccharin in training and test. In Experiment 2, again with rats that were thirsty but not hungry, stronger preferences for CS+ were observed when taste cues were presented in water in the test than when they were presented in saccharin. Addition of saccharin to the taste cues in test eliminated the preference for CS+.     

Glucocorticoids interact with the noradrenergic arousal system in the nucleus accumbens shell to enhance memory consolidation of both appetitive and aversive taste learning

It is well established that glucocorticoid hormones strengthen the consolidation of long-term memory of emotionally arousing experiences but have little effect on memory of low-arousing experiences. Although both positive and negative emotionally arousing events tend to be well remembered, studies investigating the neural mechanism underlying glucocorticoid-induced memory enhancement focused primarily on negatively motivated training experiences. In the present study we show an involvement of glucocorticoids within the nucleus accumbens (NAc) in enhancing memory consolidation of both an appetitive and aversive form of taste learning. The specific glucocorticoid receptor (GR) agonist RU 28362 (1 or 3ng) administered bilaterally into the NAc shell, but not core, of male Sprague-Dawley rats immediately after an appetitive saccharin drinking experience dose-dependently enhanced 24-h retention of the safe taste, resulting in a facilitated attenuation of neophobia. Similarly, GR agonist infusions given into the NAc shell immediately after pairing of the saccharin taste with a malaise-inducing agent enhanced memory of this negative experience, resulting in an intensified conditioned aversion. Importantly, a suppression of noradrenergic activity within the NAc shell with the β-adrenoceptor antagonist propranolol blocked the facilitating effect of a concurrently administered GR agonist on memory consolidation in both the appetitive and aversive learning task. Thus, these findings indicate that GR activation interacts with the noradrenergic arousal system within the NAc to enhance memory consolidation of emotionally arousing training experiences regardless of valence.     

Cholinergic dependence of taste memory formation: evidence of two distinct processes

Learning the aversive or positive consequences associated with novel taste solutions has a strong significance for an animal's survival. A lack of recognition of a taste's consequences could prevent ingestion of potential edibles or encounter death. We used conditioned taste aversion (CTA) and attenuation of neophobia (AN) to study aversive and safe taste memory formation. To determine if muscarinic receptors in the insular cortex participate differentially in both tasks, we infused the muscarinic antagonists scopolamine at distinct times before or after the presentation of a strong concentration of saccharin, followed by either an i.p. injection of a malaise-inducing agent or no injection. Our results showed that blockade of muscarinic receptors before taste presentation disrupts both learning tasks. However, the same treatment after the taste prevents AN but not CTA. These results clearly demonstrate that cortical cholinergic activity participates in the acquisition of both safe and aversive memory formation, and that cortical muscarinic receptors seem to be necessary for safe but not for aversive taste memory consolidation. These results suggest that the taste memory trace is processed in the insular cortex simultaneously by at least two independent mechanisms, and that their interaction would determine the degree of aversion or preference learned to a novel taste.     

Counterconditioning of an overshadowed cue attenuates overshadowing

In 3 Pavlovian conditioned lick-suppression experiments, rats received overshadowing treatment with a footshock unconditioned stimulus such that Conditioned Stimulus (CS) A overshadowed CS X. Subjects that subsequently received CS X paired with an established signal for saccharin (CS B) exhibited less overshadowing of the X-footshock association than subjects that did not receive the X-B pairings (Experiment 1). Experiment 2 replicated this effect and controlled for some additional alternative accounts of the phenomenon. In Experiment 3, this recovery from overshadowing produced by counterconditioning CS X was attenuated if CS B was massively extinguished prior to counterconditioning. These results are more compatible with models of cue competition that emphasize differences in the expression of associations than those that emphasize differences in associative acquisition.     

Exteroceptive context in tasteaversion conditioning and extinction: odour, cage, and bottle stimuli

Five experiments investigated the extent to which the exteroceptive context, present on a saccharin aversion conditioning trial with rats, controlled the resulting aversion on one-bottle extinction tests and subsequent preference tests. The presence or absence of the specific odour which had been present on the conditioning trial was found not to influence saccharin intake on extinction tests, whereas the presence of the particular compartment in which, and the bottle from which, the saccharin had been consumed greatly suppressed saccharin intake as compared to the absence of these elements. Preference tests, performed in the respective conditioning contexts, showed extinction to be specific to the compartment + bottle context: groups that had extinguished their saccharin aversion in a context different from the conditioning context, retained their aversion in the conditioning context. No such specificity was found for the odour context. However, in the absence of the taste stimulus during the extinction phase, the odour that had been present on the conditioning trial did control the amount of water consumed, whereas the compartment+bottle context did not. Moreover, on preference tests, groups that had consumed water during extinction in the presence of the odour context, evidenced a lesser saccharin aversion than groups not exposed to the odour. The results are interpreted as demonstrating that rats learn about taste, odour, cage and bottle stimuli on a taste-aversion conditioning trial, and that taste and bottle stimuli seem to be the most salient.     

The role of the insular cortex in taste function

In spite of over 30 years of research, the role of the Insular Cortex (IC) in taste memory still remains elusive. To study the role of the IC in taste memory, we used conditioned taste aversion (CTA) for two different concentrations of saccharin; 0.1% which is highly preferred, and 0.5% which is non-preferred. Rats that had been IC lesioned bilaterally with ibotenic acid (15 mg/ml) before CTA showed significant learning impairments for saccharin 0.1% but not for saccharin 0.5%. To test CTA memory retention, rats lesioned a week after CTA training became completely amnesic for saccharin 0.1% yet only mildly impaired for saccharin 0.5%. Interestingly, the resulting preference for either concentration matched that of IC lesioned animals when exposed to either saccharin solution for the first time, but not those of sham animals, implying that IC lesions after CTA for either saccharin solution rendered complete amnesia, irrespective of the original preference. Our data indicate that an intact IC is essential for CTA learning and retention, as well as for an early neophobic response, but not for taste preference itself. Our data supports a model where the IC is involved in general taste rejection.     

Toxicosis affects instrumental behaviour in rats

Three experiments examined the effect of toxicosis on instrumental responding. These studies were prompted by Morrison and Collyer's (1974, Experiment 1) finding that the induction of toxicosis after an instrumental conditioning session produces greater response suppression if the response is reinforced by a novel saccharin solution rather than familiar water during conditioning. Experiments 1 and 2 investigated whether this suppression was mediated by the Pavlovian contingency between the contextual cues and the saccharin solution or the instrumental relationship between the response and the reward. A role for the instrumental contingency was indicated by the greater suppression of the response producing novel saccharin rather than water when the context of both responses was equally associated with the saccharin and illness. Experiment 3 found that extinction of the aversion to a novel reinforcer following aversive conditioning would re-establish an action previously associated with that reinforcer, in contrast to an action whose reinforcer remained aversive. This result was a further indication that the instrumental contingency between the response and reward contributes to response suppression.     

Taste rejection of nonnutritive sweeteners in cats.

Cats reject saccharin and cyclamate and are indifferent to dulcin, although they, like other mammals, prefer sucrose. The rejection threshold for saccharin found in this experiments, .0001 M, is about 2 log steps lower than a previously reported rejection threshold for sodium saccharin. Water produces a taste in cats adapted to their own saliva. The high sodium saccharin threshold may have resulted because the taste of the sodium saccharin was masked by the taste of the water solvent; however, saccharin may also be somewhat more aversive to the cat than sodium saccharin. Saccharin may produce an aversive taste because it stimulates receptor sites sensitive to substances bitter to man as well as those sensitive to sugars. In addition, saccharin may not be an effective stimulus for all sugar-sensitive sites.     

An analysis of overshadowing and blocking

In classical aversive conditioning experiments, rats do not always learn about all aspects of a compound stimulus predicting shock. A strong stimulus may overshadow a weaker one; and pretraining on one component may block learning about a second component. These results have been explained either by appealing to a notion of selective attention, or by assuming that learning about one component is a function of prior response strength to the entire compound of which it forms a part. In Experiment I, overshadowing was demonstrated on the first trial of conditioning, i.e. before either component had acquired any response strength. In Experiment II, pretraining on one component resulted in complete failure to learn about a second component during compound training, but did not prevent additional learning about the first component. Both results were interpreted as supporting an attentional analysis of blocking and overshadowing.