Odor of Taste Stimuli in Conditioned "Taste" Aversion Learning.

The present research addresses whether rats can express odor aversions to the odor of taste stimuli. In Experiment 1, saccharin or salt were either mixed in distilled water, so the rats could taste and smell them, or presented on disks attached to the tubes' metal spouts so the rats could only smell them. Aversions were established to taste stimuli under both conditions. The results of Experiment 2 indicate that conditioning was to the odor of the tastes when they were presented on disks in Experiment 1, hence both taste and odor aversions were established by means of "taste" stimuli. Taste aversion learning thus may more properly be termed flavor aversion learning, with flavor referring to both taste and odor components. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Taste response to mixtures: Analytic processing of quality.

The tastes of 100 mM sodium chloride (NaCl), 100 mM sucrose, and 1 mM quinine hydrochloride in mixtures were investigated in golden hamsters (Mesocricetus auratus) with a conditioned taste aversion (CTA) paradigm. CTAs, established in golden hamsters by injection of lithium chloride, were quantified as percent suppression of control 1-hr stimulus intake. CTAs for 10 of 15 stimulus pairs with common components symmetrically cross-generalized, suggesting that component qualities were recognized in binary and ternary mixtures. However, CTAs to quinine were hardly learned and were weakly expressed when quinine was mixed with NaCl, and generalizations from multiple to single stimuli were stronger than vice versa (i.e., asymmetric). The behaviors reflect peripheral inhibition and/or central mixture suppression. Nonetheless, components retain their distinct qualities in mixtures, suggesting that taste processing is analytic. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Mapping Conditioned Taste Aversion Associations Using c-Fos Reveals a Dynamic Role for Insular Cortex.

Novel tastes are more effective than familiar tastes as conditioned stimuli (CSs) in taste aversion learning. Parallel to this, a novel CS-unconditioned stimulus (US) pairing induced stronger Fos-like immunoreactivity (FLI) in insular cortex (IC), amygdala, and brainstem than familiar CS-US pairing, suggesting a large circuit is recruited for acquisition. To better define the role of IC, the authors combined immunostaining with lesion or reversible inactivation of IC. Lesions abolished FLI increases to novel taste pairing in amygdala, suggesting a role in novelty detection. Reversible inactivation during taste preexposure increased FLI to familiar taste pairing in amygdala and brainstem. The difference between temporary inactivation, which blocked establishment of "safe" taste memory, and lesions points to a dual role for IC in taste learning. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Ondansetron and Delta-9-Tetrahydrocannabinol Interfere With the Establishment of Lithium-Induced Conditioned Taste Avoidance in the House Musk Shrew (Suncus murinus).

Considerable evidence suggests that rats can learn to avoid a taste in the absence of nausea. The current experiments evaluated the potential of the antiemetic agents, ondansetron (OND) and delta-9-tetrahydrocannabinol (THC), to interfere with lithium chloride (LiCl)-induced taste avoidance in the house musk shrew, Suncus murinus, an insectivore that, unlike rats, is capable of vomiting. At a dose that did not modify saccharin (Experiment 1) or sucrose (Experiment 2) intake, OND prevented the establishment of LiCl-induced taste avoidance in the shrew. A low dose of THC (1 mg/kg), which did not modify sucrose intake during conditioning, also prevented the establishment of LiCl-induced taste avoidance in the shrew. Higher doses of THC were also effective, but they also suppressed sucrose consumption during conditioning. These results suggest that nausea is a necessary component of the unconditioned stimulus for the establishment of conditioned taste avoidance in the shrew, unlike the rat, which does not vomit when injected with a toxin. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Re-examination of amphetamine-induced conditioned suppression of tastant intake in rats: The task-dependent drug effects hypothesis.

This study reexamined Grigson's reward comparison hypothesis (1997), which claimed to have resolved the paradox of addictive, rewarding drugs manifesting an aversive effect in the conditioned taste aversion (CTA) paradigm. Here, the authors compared the conditioned suppression effects of lithium chloride (LiCl) and amphetamine in a series of three experiments. In Experiment 1, the concentrations of saccharin solution (conditioned stimulus [CS]) and the doses of amphetamine or LiCl (unconditioned stimulus [US]) were manipulated. In Experiment 2, the effects of employing backward versus forward pairings of the CS and US were compared. Finally, in Experiment 3, the additivity of amphetamine's reward property and LiCl's aversive property was examined. The results of these experiments, respectively, indicated that: (1) manipulating saccharin solution concentrations does not distinguish the suppression effect caused by rewarding or aversive effects when amphetamine or LiCl served as the US; (2) both backward and forward pairings produced suppression of saccharin solution intake regardless of whether amphetamine or LiCl was used as the US; and (3) combining amphetamine and LiCl did not diminish the suppression effect, as would be expected if they had opposing mechanisms for the effects; instead, an additive effect occurred. Taken together, these results suggest that the drug of abuse amphetamine and the emetic drug LiCl both possess aversive properties in the CTA paradigm. No rewarding effects of amphetamine were detected in our experimental data. In all, our results do not support the Grigson's reward comparison hypothesis (1997) and a new "task-dependent drug effects hypothesis" is proposed. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

5,7-Dihydroxytryptamine Lesions of the Dorsal and Median Raphe Nuclei Interfere With Lithium-Induced Conditioned Gaping, but Not Conditioned Taste Avoidance, in Rats.

Both the dorsal and median raphe nuclei of the midline brainstem region in rats were lesioned with the neurotoxin 5,7-dihydroxytryptamine. Rats were then surgically implanted with intraoral cannulas for fluid delivery and received a single conditioning trial in which 2-min saccharin infusion was followed by either lithium or saline administration. The conditioned gaping seen in the lithium-conditioned rats was significantly attenuated by raphe lesions, indicating that reduction of forebrain serotonin levels interferes with conditioned gaping. However, lesioned rats still expressed comparable conditioned taste avoidance as measured by both the 1- and 2-bottle consumption tests. These results parallel previous pharmacological findings indicating that reduction of serotonin activity interferes with conditioned gaping, but not conditioned taste avoidance. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Effects of neuropeptide Y on feeding microstructure: Dissociation of appetitive and consummatory actions.

The effects of intracerebroventricular application of Neuropeptide Y (NPY) on licking microstructure for sucrose, saccharin, and water solutions were evaluated. In Experiment 1, NPY increased meal size for three sucrose concentrations (0.03 M, 0.3 M, and 1.0 M) by increasing licking burst number but not size and by extending meals more than four-fold in duration with a slow, sustained rate of ingestion in late phases of the meal. Results are consistent with the interpretation that NPY suppressed inhibitory postingestive feedback. Experiment 2 supported this conclusion. NPY significantly increased the number of meals initiated for water, 0.1% saccharin, and 1.0 M sucrose solutions, but meal size was only increased for 1.0 M sucrose. Therefore, NPY also increased appetitive feeding behaviors, but its consummatory effects were limited to caloric solutions. The results are discussed with regard to their potential to explain current discrepancies in the literature. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Once is too much: Conditioned changes in accumbens dopamine following a single saccharin-morphine pairing.

The present study tested whether presentation of a taste cue would support conditioned suppression of dopamine in the nucleus accumbens (NAcc) following a single taste-drug pairing. Nondeprived male Sprague-Dawley rats were given 20-min access to a 0.15% saccharin conditioned stimulus (CS). Immediately thereafter, experimental rats were injected with morphine (15 mg/kg ip); standard controls were injected with saline; and explicitly unpaired controls were injected with morphine, but approximately 24 hr later. All rats were then given one 20-min CS-only test. Microdialysis samples from the NAcc were measured over 20-min intervals before, during, and after CS access on the conditioning and test trial. The results showed that a single saccharin-morphine pairing led to a marked reduction in CS intake, and the reduction in intake was accompanied by a conditioned blunting of the accumbens dopamine response to the saccharin reward cue. In turn, a single exposure to the saccharin cue also blunted the unconditioned dopamine response to morphine. Reward comparison effects, then, are cross-modal, bidirectional, and immediate, resulting in both unconditioned and conditioned changes in brain and behavior. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Genetic Correlational Analyses of Ethanol Reward and Aversion Phenotypes in Short-Term Selected Mouse Lines Bred for Ethanol Drinking or Ethanol-Induced Conditioned Taste Aversion.

Short-term selective breeding created mouse lines divergent for ethanol drinking (high drinking short-term selected line [STDRHI], low drinking [STDRLO]) or ethanol-induced conditioned taste aversion (CTA; high [HTA], low [LTA]). Compared with STDRLO, STDRHI mice consumed more saccharin and less quinine, exhibited greater ethanol-induced conditioned place preference (CPP), and showed reduced ethanol stimulation and sensitization under some conditions; a line difference in ethanol-induced CTA was not consistently found. Compared with LTA, HTA mice consumed less ethanol but were similar in saccharin consumption, sensitivity to ethanol-induced CPP, and ethanol-induced locomotor stimulation and sensitization. These data suggest that ethanol drinking is genetically associated with several reward-and aversion-related traits. The interpretation of ethanol-induced CTA as more genetically distinct must be tempered by the inability to test the CTA lines beyond Selection Generation 2. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

The state of the reward comparison hypothesis: Theoretical comment on Huang and Hsiao (2008).

Rats avoid intake of a gustatory cue following pairings with a drug of abuse, such as morphine or cocaine. Despite the well-established rewarding properties of these drugs, the reduction in intake of the taste cue has been interpreted as a conditioned taste aversion for decades. In 1997, I proposed the reward comparison hypothesis suggesting that rats avoided intake of the drug-associated taste cue because the value of the taste cue pales in comparison to the highly rewarding drug of abuse expected in the near future. In this issue of Behavioral Neuroscience, A. C. W. Huang and S. Hsiao (see record 2008-17011-002) challenge the reward comparison hypothesis by showing parallels between amphetamine and LiCl-induced suppression of CS intake. This commentary addresses the current state of the reward comparison hypothesis in the context of the experiments completed by Huang and Hsiao and their new task-dependent drug effects hypothesis. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Dissociating the conditioning and the anorectic effects of estradiol in female rats.

The present series of experiments challenges the ability of the hormone estradiol to act as an unconditioned stimulus in the conditioned taste avoidance (CTA) learning paradigm. We hypothesize that reductions in sucrose consumption observed after pairing it with estradiol are not indicative of associative learning, but due to the unconditioned expression of estradiol’s anorectic effects during the time of CTA assessment. Three experiments in which a sucrose solution was paired with estradiol were conducted to test this hypothesis. Experiment 1 demonstrated that female rats expressed a reduction in post-pairing sucrose consumption even though the anorectic effects of estradiol had subsided. Experiment 2 showed that although a low dose of estradiol produced anorexia, it did not elicit post-pairing reductions in sucrose consumption. Experiment 3 revealed that contingent pairing was a requirement for post-pairing reduction in sucrose consumption even when testing was done at a time when anorexia is expressed. These findings demonstrate the dissociability of the conditioning and anorectic effects of estradiol, providing evidence against the hypothesis. The results are discussed in terms of independent neural mechanisms underlying the disparate behaviors. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Comparison of nutritive and nonnutritive stimuli in intestinal and oral conditioned taste aversion paradigms.

The intestinal taste aversion paradigm has previously demonstrated that animals could orally discriminate between carbohydrate and fat subsequent to pairing a gastrointestinal (GI) infusion of 1 nutrient with lithium chloride (LiCl), whereas they could not discriminate between 2 nonnutritive flavors (A. L. Tracy, R. J. Phillips, M. M. Chi, T. L. Powley, & T. L. Davidson, 2004). The present experiments assessed the relative salience of nutritive and nonnutritive stimuli when presented either intestinally or orally. Two compound stimuli, each comprising 1 nutrient and 1 nonnutritive flavor, were presented in training and were paired with LiCl or saline. Subsequent oral intake of the nutrients alone, the flavors alone, or the compounds was measured. Results showed that rats discriminated both nutrients and flavors independently after GI or oral training, whereas the compounds were discriminated only after oral training, indicating substantive differences in the processing of these stimuli. This suggests that nutrient activation of the GI tract may potentiate learning about nonnutritive flavors analogously to taste-potentiated odor conditioning. The ability to learn about the oral properties of stimuli in the GI tract suggests a new account of delayed taste aversion learning as well as learning about the positive nutritive consequences of food consumption. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Dissociation of Temporal Dynamics of Heart Rate and Blood Pressure Responses Elicited by Conditioned Fear but Not Acoustic Startle.

Fear-inducing stimuli were hypothesized to elicit fast heart rate (HR) responses but slow mean arterial blood pressure (MAP) responses and thus were studied in auditory fear conditioning and acoustic startle at high temporal resolution in freely moving mice and rats. Fear-induced instantaneous acceleration of HR reaching maximum physiological values and subsequent recovery to baseline were observed. The MAP response consisted of an immediate, mild, and transient increase followed by a sluggish, profound elevation and slow recovery. HR and MAP responses served as reliable indicators of conditioned fear in mice with dissociated temporal dynamics. Unconditioned auditory stimuli, including acoustic startle stimuli, elicited only fast, mild, and transient MAP and HR elevations in mice and rats, reflecting arousal and attention under these experimental conditions. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Making time count: Functional evidence for temporal coding of taste sensation.

Although the temporal characteristics of neural responses have been proposed as a mechanism for sensory neural coding, there has been little evidence thus far that this type of information is actually used by the nervous system. Here the authors show that patterned electrical pulses trains that mimic the response to the taste of quinine can produce a bitterlike sensation when delivered to the nucleus tractus solitarius of behaving rats. Following conditioned aversion training using either "quinine simulation" patterns of electrical stimulation or natural quinine (0.1 mM) as a conditioned stimulus, rats specifically generalized the aversion to 2 bitter tastants: quinine and urea. Randomization of the quinine simulation patterns resulted in generalization patterns that resembled those to a perithreshold concentration (0.01 mM) of quinine. These data provide strong evidence that the temporal pattern of brainstem activity may convey information about taste quality and underscore the functional significance of temporal coding. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Temporal Coding of Sensation: Mimicking Taste Quality With Electrical Stimulation of the Brain.

Two experiments suggested that the temporal pattern of a taste response in the brain can convey meaningful information. In Experiment 1, rats avoided lick-contingent electrical stimulation of the nucleus of the solitary tract (NTS; the first synaptic relay for taste) when the temporal pattern of pulses mimicked the electrophysiological response to quinine, but not when the temporal pattern was randomized. In Experiment 2, rats avoided lick-contingent electrical stimulation of the NTS that mimicked the temporal pattern of a sucrose response following stimulation-illness pairings. This aversion generalized to sucrose but not to the other tastants; extinction of the aversion to electrical stimulation also extinguished the aversion to sucrose. Results replicate and extend previous findings (P. M. Di Lorenzo & G. S. Hecht, 1993). (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Temporal Coding in Conditioned Inhibition: Analysis of Associative Structure of Inhibition.

Two experiments with rats as subjects were conducted to investigate the associative structure of temporal control of conditioned inhibition through posttraining manipulation of the training excitor-unconditioned stimulus (US) temporal relationship. Experiment 1 found that following simultaneous Pavlovian inhibition training (i.e., A → US/XA-no US) in which a conditioned stimulus (CS A) was established as a delay excitor, maximal inhibition was observed on a summation test when CS X was compounded with a delay transfer CS. Furthermore, posttraining shifts in the A-US temporal relationship from delay to trace resulted in maximal inhibition of a trace transfer CS. Experiment 2 found complementary results to Experiment 1 with an A-US posttraining shift from serial to simultaneous. These results suggest that temporal control of inhibition is mediated by the training excitor-US temporal relationship. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Effects of Selective Lingual Gustatory Deafferentation on Suprathreshold Taste Intensity Discrimination of NaCl in Rats.

In rats, chorda tympani nerve transection (CTX) greatly increases the detection threshold of sodium chloride (NaCl) and severely disrupts salt discriminability. Here it is shown that CTX has surprisingly little effect, if any, on suprathreshold intensity discrimination. Glossopharyngeal nerve transection (GLX), which has no reported effect on salt sensibility, also did not affect performance. Rats were tested in a 2-response, operant taste intensity discrimination task. Difference thresholds for CTX rats were only slightly higher (-0.15 log/10 unit) than those for GLX and sham-transected rats, when 0.05 M served as the standard, and did not significantly differ when 0.1 M NaCl was the standard. Although the perceived intensity of NaCl might be reduced by CTX, input from remaining taste nerves sufficiently maintains the relative discriminability of suprathreshold NaCl concentrations. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Psychophysical Investigations of Cetylpyridinium Chloride in Rats: Its Inherent Taste and Modifying Effects on Salt Taste.

Salts are transduced by at least 2 mechanisms: (a) antagonized by amiloride and (b) antagonized by cetylpyridinium chloride (CPC). The authors report on 4 behavioral experiments in rats that characterize the orosensory properties of CPC itself as well as its effect in suppressing the intensity of NaCl and KCl taste. Experiments 1 and 2 indicated that CPC has a quinine-like taste quality. Experiments 3 and 4 demonstrated that the recognition of KCl, but not NaCl, is modestly reduced by mixture with CPC. However, control experiments call into question the mechanism of the salt suppression of CPC, because both CPC-salt and quinine-salt mixtures had similar effects. The relevance of these studies for understanding salt and bitter taste coding is discussed. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Enhanced discrimination between flavor stimuli: Roles of salience modulation and inhibition.

Rats were given intermixed preexposure to the compound flavors AX and BX and to the compound CX in a separate block of trials (4 presentations of each compound). In Experiment 1, rats showed less generalization of conditioned aversion from AX to BX than from CX to BX, a perceptual learning effect. Experiment 2 showed that the formation of an excitatory association proceeded more readily between A and B than between C and B, suggesting that intermixed preexposure maintains the effective salience of A and B and does not establish inhibition between them, a process that would require prolonged preexposure. According to this analysis, salience modulation and associative inhibition may contribute to perceptual learning at different stages of preexposure. (PsycINFO Database Record (c) 2010 APA, all rights reserved)