A within-event analysis of taste-potentiated odor and contextual aversions

Taste-potentiated odor aversion learning in rats with lesions of the insular cortex

The current study assessed the influence of excitotoxic lesions of the insular cortex (IC) on taste-potentiated odor aversion (TPOA) learning. Water-deprived rats initially received a single odor-toxicosis or odor/taste-toxicosis pairing and were subsequently tested, in separate trials, with the odor and the taste stimulus. Indicating TPOA, neurologically intact rats conditioned with the odor/taste compound stimulus acquired significantly stronger odor aversions than normal rats conditioned with the odor stimulus. IC lesions disrupted TPOA, conditioned taste aversion and taste neophobia. The finding that taste did not potentiate odor aversion learning in the IC-lesioned rats provides support for the "within-compound association" analysis but is inconsistent with the "sensory-and-gate" account of TPOA learning.

Within-compound associations are not sufficient to produce taste-mediated odor potentiation

Rats were used in two flavor-aversion experiments to determine if within-compound associations could be detected with a taste+odor compound that would not support taste-mediated odor potentation. In Experiment 1, following taste+odor compound conditioning, postconditioning taste extinction significantly weakened the odor aversion. In Experiment 2, following taste+odor compound conditioning, postconditioning taste inflation significantly strengthened the odor aversion. There was no evidence that taste potentiated the odor aversion in either Experiment 1 or 2. Thus, the results demonstrate that the presence of within-compound associations is not sufficient to produce taste-mediated odor potentiation. We offer a mediated conditioning explanation to account for the results of these two experiments.

Perceptual Learning Enhances Retrospective Revaluation of Conditioned Flavor Preferences in Rats

Forward and backward blocking of taste preference learning was compared in rats. In the forward condition, thirsty rats were exposed to a flavor (A) in sucrose solution (+) or in water (-), after which they were exposed to A in compound with another flavor (B) in sucrose solution (i.e., AB+). In the backward condition, these phases were reversed. Consumption of B alone was assessed when rats were food deprived. In the forward condition, rats given A+ consumed less B than rats given A-, providing evidence of forward blocking, whereas in the backward condition, rats given A+ drank more of B than those given A-. Subsequent experiments found that alternating but not blocked preexposure to A and B, when given prior to training, produced blocking of B whether A+ was given before or after AB+, suggesting that prior failures to observe backward blocking reflect failures of discrimination.

Ontogenetic differences in the expression of conditioned visual aversions

We examined ontogenetic differences in the expression of conditioned visual aversions. Sprague-Dawley-derived rats, 16 or 21 days of age, were conditioned with either an element (brightness) or compound (brightness/odor) CS+ and tested for their aversion to the common element (brightness). Aversions to the brightness cue were assessed by either a traditional test of preference between the CS+ brightness and a contrasting brightness or by assessment of freezing in the presence of either brightness cue. The results indicated that strength of conditioning as well as the expression of overshadowing/potentiation was dependent on the age of the animal and on the technique used to assess conditioning.

Prefrontal cortex and the relative associability of taste and place cues in rats

The effects of cutting the corticocortical connections between medial and sulcal prefrontal areas on the conditioning of taste and place cues were examined. In Experiment 1, rats were simultaneously exposed to taste and place cues before injection of 0.15 M LiCl. In controls, a significant conditioned taste aversion (CTA) but no conditioned place aversion (CPA) was observed. In contrast, rats with bilateral knife cuts showed a significant CPA but a weaker CTA. To test whether these results could have been due to the effects of simultaneously exposing the rats to taste and place cues during conditioning, rats were trained independently in either CTA or CPA paradigms in Experiment 2. In the CTA test, rats both in operated and control groups showed a CTA when first tested. Rats with bilateral knife cuts, however, showed a weaker CTA than those in the control group. In the CPA test, rats in the control group did not exhibit a CPA, whereas the knife cut group did. Rats with sham lesions tested in Experiment 2 did not differ from control subjects on either the CPA or the CTA test. Thus, bilateral cuts increased the CPA and decreased the CTA even when tested independently. These results indicate that the relative ease of association of place and taste stimuli may be accounted for in part by the organization of the intrinsic connections of the prefrontal cortex in the rat.

Olfactory and gustatory stimuli in food-aversion learning of rats

Two studies of food-aversion learning in the rat examined some crucial conditions for the potentiation of an odor component by a novel taste. Wistar rats were exposed twice to an odor-taste compound and then were treated either immediately or the next day with lithium chloride, a noxious substance. Immediately poisoned rats acquired an aversion to the odor component when it was paired with a familiar taste. Only in one experimental condition did some weak conditioning occur: when the new odor was presented together with a new taste. Thus, the classical phenomenon of overshadowing of the less salient odor component by the more salient taste was observed in all of our studies. This was true for situations in which the odor was presented near the drinking spout as well as for situations in which the odor was presented at some distance in front of the drinking source (Experiment 1). In addition, the weaker an odor component was, the more easily it was overshadowed by the taste (Experiment 2). All results fit well into classical rules of Pavlovian conditioning and fail to support the assumption that special mechanisms exist in food-related odor conditioning in rats.

Taste-potentiated odor aversion learning based on amphetamine

This study sought to determine whether a taste can potentiate a conditioned odor aversion based on amphetamine as well as those based on lithium. A taste-potentiated odor aversion (TPOA) based on lithium was obtained in Experiment 1 only with a low concentration of an almond odor. This concentration was used in Experiment 2 where the taste, 0.1% saccharin, potentiated an odor aversion based on 1 mg/kg d-amphetamine. This was replicated in Experiment 3 where potentiation was found with doses of both 1 and 3 mg/kg amphetamine, and no effect of dose was detected. It was concluded that TPOA learning is not restricted to drugs such as lithium that produce conditioned unpalatability as well as conditioned aversions to a taste, because amphetamine does not produce conditioned unpalatability at the doses used here. Furthermore, because in Experiment 3 postconditioning extinction of the saccharin aversion removed the potentiation effect, it appears that this form of TPOA may depend on an association between the odor and taste, as proposed by within-compound theory.

Ethanol-mediated taste aversions and state-dependency in preweanling (16-day-old) rats

Requirements for conditioning of an ethanol-mediated taste aversion in 16-day-old rat pups were examined. Experiment 1 demonstrated that preweanling rats are capable of acquiring, in two trials, an aversion to a 15% sucrose solution when followed by intragastric intubation of a 1.2 g/kg dose of 17% v/v ethanol, but not when followed by a 0.4 g/kg dose. Comparison was with control animals given sucrose followed by an equivalent volume isocaloric Half and Half. When the 0.4 g/kg dose of ethanol preceded sucrose presentation by 30 min (Experiment 2), the aversion was learned, suggesting that the effective delay between the sucrose and the critical consequences of the ethanol had been too long with the former procedure. Expression of the sucrose aversion required, however, the reinstatement of the context of intoxication--state-dependent retention. Finally, the results of Experiment 3B indicated that, in addition to the association between the sucrose and the aversive consequences of alcohol intoxication, the orosensory cues resulting from alcohol's direct elimination, via such processes as respiration and salivation, became associated with the appetitive properties of the sucrose. This was evidenced by a conditioned increase in preference for ethanol odor. Possible age-related differences in the ability to associate stimuli with alcohol's unconditioned consequences, and in state dependency are discussed.

Conditioning of morphine-induced taste aversion and analgesia

The process of selective associations is evident in the aversive conditioning literature, where it has been shown that external cues are readily associated with peripheral pain, whereas taste cues are more easily associated with effects of drug administration. Within this framework, it is of interest that the failures to obtain a conditioned analgesic response to a morphine-associated CS have used external cues as conditioned stimuli. In Experiment 1, subjects re-exposed to a morphine-associated CS not only expressed the anticipated taste aversion, but also exhibited a decrease in pain sensitivity that was evident 15 or 30 min following CS re-exposure. Experiment 2 suggested that the conditioned analgesic response was opioid mediated, as pre-test administration of naloxone blocked expression of the analgesic CR. In Experiment 3, an increase in opiate receptor sensitivity produced by chronic naltrexone treatment did not affect the strength of the taste aversion, but resulted in an increase in the magnitude of the conditioned analgesic response. Collectively, these data suggest a neuropharmacological dissociation in systems mediating the two responses.

A potential role of saline trials in morphine-induced place-preference conditioning

The necessary conditions to alter rats' initial preferences for two sides of a shuttlebox were investigated, using procedures that are often used in the study of drug reinforcement. In Experiment 1, pairings of morphine sulfate (15 mg/kg, intraperitoneally) and either the nonpreferred side or a holding box was factorially combined with alternate-day pairings of saline and either the preferred side or a holding box. Pairings of saline and the preferred side were necessary and sufficient to increase preferences for the initially nonpreferred side. In Experiment 2, pairings of saline and the nonpreferred side, but not the holding box, strengthened the initial preference, regardless of whether morphine or saline injections preceded alternate-day holding-box placements. In Experiment 3, injection and placement in the preferred side in an unpaired manner, or placement only, decreased preferences for that side more than saline injections alone or no treatment. Paired saline injections and placement produced a greater change in preference than no treatment.