Conditioned taste aversion to injected flavor: differential effect of anesthesia on the formation of the gustatory trace and on its association with poisoning in rats

Non-nicotine constituents in e-cigarette aerosol extract attenuate nicotine's aversive effects in adolescent rats

BACKGROUND

Development of preclinical methodology for evaluating the abuse liability of electronic cigarettes (ECs) in adolescents is urgently needed to inform FDA regulation of these products. We previously reported reduced aversive effects of EC liquids containing nicotine and a range of non-nicotine constituents (e.g., propylene glycol, minor tobacco alkaloids) compared to nicotine alone in adult rats as measured using intracranial self-stimulation. The goal of this study was to compare the aversive effects of nicotine alone and EC aerosol extracts in adolescent rats as measured using conditioned taste aversion (CTA), which can be conducted during the brief adolescent period.

METHODS AND RESULTS

In Experiment 1, nicotine alone (1.0 or 1.5 mg/kg, s.c.) produced significant CTA in adolescent rats in a two-bottle procedure, thereby establishing a model to study the effects of EC extracts. At a nicotine dose of 1.0 mg/kg, CTA to Vuse Menthol EC extract, but not Aroma E-Juice EC extract, was attenuated compared to nicotine alone during repeated two-bottle CTA tests (Experiment 2a). At a nicotine dose of 0.5 mg/kg, CTA to Vuse Menthol EC extract did not differ from nicotine alone during the first two-bottle CTA test but extinguished more rapidly across repeated two-bottle tests (Experiment 2b).

CONCLUSIONS

Non-nicotine constituents in Vuse Menthol EC extracts attenuated CTA in a two-bottle procedure in adolescents. This model may be useful for anticipating the abuse liability of ECs in adolescents and for modeling FDA-mandated changes in product standards for nicotine or other constituents in ECs.

c-Fos activity in the insular cortex, nucleus accumbens and basolateral amygdala following the intraperitoneal injection of saccharin and lithium chloride

This study examined c-Fos expression in selected brain areas consequent to intraperitoneal (IP) administration of saccharin and lithium chloride. Rats were tested for aversion to the saccharin as measured by flavor consumption and orofacial reactions in the taste reactivity (TR) test. It was found that intraperitoneal conditioning resulted in the reduction in voluntary consumption but not in the production of aversive orofacial responses to the saccharin. The immunohistochemistry quantification revealed increased c-Fos activity in the insular cortex, the shell and core regions of the nucleus accumbens, and the basolateral nucleus of the amygdala. These results show that a conditioned taste aversion can be induced without direct oropharyngeal gustatory stimulation at the time of conditioning. In addition, this study provide evidence of increased neural activity in response to intraperitoneal saccharin injections.

Ondansetron blocks LiCl-induced conditioned place avoidance but not conditioned taste/flavor avoidance in rats

The ability of an experimental agent to support conditioned taste/flavor avoidance (CT/FA) in rats often is interpreted as sufficient evidence that the agent produced a state of malaise or nausea. Paradoxically, however, CT/FA also is induced by certain drugs that support conditioned preferences in rats, suggesting that CT/FA is insufficient to reveal a negative hedonic state. The present study tested the hypothesis that the anti-nausea drug ondansetron (OND) would block the ability of nauseogenic lithium chloride (LiCl) to support conditioned place avoidance (CPA), without attenuating LiCl-induced CT/FA. After pre-treatment with either OND or vehicle, rats were conditioned with i.p. injection of 0.15 M LiCl containing 2% saccharin (LiCl+sac) on conditioning day 1, and with 0.15 M NaCl alone on conditioning day 2. Rats were confined to a distinct chamber of a CPA apparatus after each conditioning injection. In other rats, OND or vehicle pre-treatment was followed by NaCl+sac on conditioning day 1, and LiCl alone on day 2. Subsequent testing revealed that OND blocked the ability of LiCl to support CPA. Conversely, in the same rats, OND did not alter the ability of LiCl to condition avoidance of 0.2% sac solution during a 60 min bottle test. In a separate experiment, a sensitive 2-bottle choice test was used to confirm that OND pre-treatment does not reduce the ability of LiCl to support CT/FA. These results support the view that CPA is an additional useful tool to reveal the experience of malaise and nausea in rats, whereas CT/FA demonstrated in bottle intake tests is insufficient for this purpose.

Ketamine blocks the formation of a gustatory memory trace in rats

N-methyl-D-aspartate (NMDA) receptors appear to play a central role in learning and memory processes, as the administration of antagonistic substances of these receptors hinders learning acquisition by using different behavioral paradigms (e.g., Riedel G, Platt B, Micheau J. Glutamate receptor function in learning and memory. Behavioural Brain Research, 2003;140 (1-2):1-47.). In the specific case of conditioned taste aversion, the administration of ketamine seems to affect the acquisition of conditioning when the drugs are administered before the experimental treatment. In this paper we present three experiments designed to analyze the effect of different ketamine doses (25 mg/kg, 50 mg/kg, 75 mg/kg and 120 mg/kg), administered between exposure to a taste (the conditioned stimulus) and the administration of the unconditioned stimulus, on the acquisition of a taste aversion association. The results reveal that higher ketamine doses (75 mg/kg and 120 mg/kg) have a disruptive effect on conditioned taste aversion by impeding the formation of the gustatory trace.

Hypothermia modifies the effective CS-US interval in conditioned taste aversion in rats

Conditioned taste aversion can be acquired when rats experience an unconditioned stimulus (US) while anesthetized. In contrast to anesthetics, a hypothermia-induced comatose state immediately after presentation of a taste conditioned stimulus (CS) prevented a taste-illness association at relatively short CS-US intervals and potentiated the aversion at longer intervals. Results at shorter CS-US intervals were explained on the basis of hypothermia's temporally graded amnesitc properties. Evidence for conditioning at the longer intervals was discussed in relation to slowing down metabolism allowing for associations to be formed at CS-US intervals that normally do not result in evidence of conditioning. Manipulating body temperature during the CS-US interval was demonstrated to alter rats' ability to bridge temporal gaps in associative learning.

Ketaset-Rompun extends the effective interstimulus interval in long-trace taste-aversion conditioning in rats

The effect of anesthesia (Ketaset-Rompun) interpolated between the conditioned stimulus (CS) and unconditioned stimulus (US) during long-trace taste-aversion conditioning in rats was examined in three experiments. In Experiment 1, rats that were anesthetized immediately after experiencing a saccharin solution formed a taste aversion at a 3-h interval that typically does not support conditioning, a prolongation effect. Prior experience with the anesthetic eliminated the associability of the aversive consequences of the anesthetic but did not eliminate the anesthetic's prolongation effect. Some evidence was also obtained indicating that LiCl produced an aversion at the 3-h interval in unanesthetized rats if they had experience with the anesthetic prior to conditioning. In Experiment 1a, the interval between prior experience and conditioning was extended from 24 to 96 h. Results demonstrated that the evidence for conditioning at 3 h for unanesthetized subjects in Experiment 1 was not a robust finding. By reversing the role of Ketaset-Rompun (KR) and LiCl as prior experience manipulation and US treatment in Experiment 2, the prolongation effect was shown not to be due to the summation of the aversive properties of the anesthetic and the LiCl. Results were interpreted in terms of a hypothesized metabolic pacemaker.

CS-US delay does not impair appetitive conditioning in Chasmagnathus

Habituation and appetitive conditioning have been already described in the crab Chasmagnathus. The purpose of this work is to study whether associative learning can be obtained despite a long conditioned stimulus-unconditioned stimulus interval. Results of the first experiment show that the weakening of temporal contiguity does not prevent appetitive conditioning to occur while after a long 4-h delay, conditioning wanes completely. A second experiment was conducted, after one and three days of training respectively, confirming the above results. Though initially neutral the context trace may be still available immediately after training and for the period of two but not after 4:00 h, demonstrating a forward limit for the conditioning window. After 3 days of training, a further decrease in the exploratory activity suggested that a longer training could increase the relative weight of habituation. Conditioning and habituation seem to work as opponent processes in the crab CHASMAGNATHUS GRANULATUS: if habituation training in the box is followed by the administration of reinforcement after a short period of time, appetitive conditioning will take place. However, as this interval is increased, habituation prevails. A persistent effect of the exposure to a given environment that may underlie trace conditioning in this crab is discussed in adaptive terms.

Effects of conditioned food aversions on nutritional behavior in humans

Conditioned food aversion (CFA) and taste aversion (CTA) are widely occurring phenomena mediating rejection of solids or liquids, the ingestion of which has induced the onset of post-ingestional malaise. It is a powerful and durable imprint learning that may influence food choice and intake in all animals, including humans. For ethical reasons, CTA has been extensively investigated in a wide variety of laboratory animal's species but only incidentally in humans. Nevertheless, convincing evidence has been provided that CFA and CTA learning are possible in a wide range of human subjects. The results in humans may have some limitations in accuracy since data are sparse, sometimes indirect, and poorly controlled. There is only limited information on the extent of CFA in the elderly since most studies have employed questionnaire and/or interview methods on young people (i.e. college students). The present review evaluates the literature derived both from laboratory animals and humans. In the first instance, the salient features of food and taste aversion learning and the neural mechanisms involved in this learning behavior will be examined. Then, the problems encountered when trying to assess the role of learned food and taste aversions in the nutritional status of healthy as well as sick young or elderly people will be considered. In particular, the importance of CFA on the nutritional status of cancer patients and treatment of alcoholism will be examined. It is concluded that the data are compelling enough to warrant further research and, some indications and recommendations are suggested.

Neural substrates for conditioned taste aversion in the rat

Conditioned taste aversions (CTAs) are well known to be robust and long-lasting instances of learning induced by a single CS (taste)-US (malaise) pairing. CTA can be taken as a general model to search for neural mechanisms of learning and memory. In spite of extensive research on CTAs using a variety of approaches during the last three decades, the neural mechanisms of taste aversion learning still remain unsolved. In this article we propose a model of neural substrates of CTAs on the basis of our recent studies incorporating previous findings by other workers. Our studies mainly included experiments using ibotenic acid injections into various parts of the rat brain as a lesion technique, and c-fos immunohistochemistry in naive and CTA trained rats. CTAs were established by pairing the ingestion of saccharin (CS) with an ip injection of LiCl (US). Behavioral studies have shown that the parabrachial nucleus (PBN), medial thalamus, and basolateral nucleus of the amygdala are essential for both acquisition and retention of CTAs. C-fos studies suggested that association between gustatory CS and visceral US takes place in the PBN. The gustatory cortex (GC) may modify the strength of this association depending on the nature of the CS, viz., novel or familiar. The amygdala is indispensable for the expressions of CTAs. Tastes with hedonic values are stored in the GC in a long-term manner.

Distribution of lithium in the rat brain after a single administration known to elicit aversive effects

In order to get insight into the brain areas involved in the initial unpleasant effects resulting from the administration of lithium (Li+), its distribution was mapped in the rat brain using the 6Li(n,alpha)3H nuclear reaction after a single injection of 6Li2SO4 at doses and latencies corresponding to the elicitation of such unpleasant effects. An improved method for visualization and measurement of local Li+ concentrations was used consisting in diffracting light along the tracks left by alpha particles in a dielectric detector. The distribution of Li+ was found less homogeneous than when Li+ was administered chronically. Periaqueductal and periventricular structures were the brain areas containing the highest concentrations of Li+.

Saccharin's aversive taste in rats: evidence and implications

In most biobehavioral research using rats as subjects, saccharin is viewed as a "sugar substitute"--sweet, palatable, and conveniently lacking in calories. This characterization has merit: Saccharin seems to share some sensory, affective, and motivating properties with sugars. This paper focuses on saccharin's lesser known, aversive properties. Evidence of saccharin's distinctive taste in rats is reviewed, followed by discussion of several modulating variables. Procedures that influence rats' responses to saccharin and their measurement are summarized, and the argument is advanced that saccharin can be used to study the relationship of taste and ingestion to learning, stress, and emotional processes in rats and humans.

Acquisition of conditioned taste aversion in rats is prevented by tetrodotoxin blockade of a small midbrain region centered around the parabrachial nuclei

A remarkable feature of conditioned taste aversion (CTA) is the resistance of the association between the gustatory trace and symptoms of poisoning against disruptive procedures. In an attempt to identify the neural substrate of this phase of CTA acquisition, thirsty rats were offered 0.1% saccharin for 15 min, were immediately afterwards anesthetized with pentobarbital, received stereotaxic injections of tetrodotoxin (TTX, 10 ng/microliters) into various brainstem regions and were poisoned with IP injection of LiCl (0.15 M, 2% body weight). In Experiment 1, TTX prevented CTA acquisition when injected into the parabrachial nuclei but was ineffective in the lower medulla. TTX alone did not elicit CTA even at brain sites in which it caused death in 30% of the animals. In Experiment 2, the brainstem was systematically explored by a grid of bilateral TTX injections. A spatial gradient of the CTA disruption pointed to the parabrachial nuclei as the brain region responsible for the amnesic effect observed. Experiment 3 showed that a single TTX injection into the parabrachial nucleus on one side did not prevent CTA acquisition and that similarly ineffective were TTX injections in the sagittal plane both at the mesencephalic and bulbar levels. It is concluded that the pivotal role of the parabrachial nuclei in the formation of the permanent CTA engram can only be revealed by functional blockade which is more radical than that achieved during general anesthesia.