Conditioned Taste Aversion Learning Research Articles

Molecular Signaling during Taste Aversion Learning

Behavioral and neural assessment tools have been used to identify cellular and molecular events that occur during taste aversion acquisition. Studies described here include an assessment of taste information processing and taste-illness association using fos-like immunoreactivity (FLI) to mark populations of cells that react strongly to the taste conditioned stimulus (CS), the illness unconditioned stimulus (US), or the pairing of CS and US. Exposure to a novel, but not a familiar, CS taste (saccharin) was found to induce robust increases in FLI in some, but not all, brain regions previously implicated in taste processing or taste aversion learning. Striking effects of taste novelty on FLI were found in central amygdala (CNA) and insular cortex (IC) but not in basolateral amygdala (BLA), pontine parabrachial nucleus (PBN), or nucleus of the solitary tract (NTS). Of those regions responding to taste novelty, only CNA showed significant elevations in FLI in response to the US, LiCl. In additional studies, FLI was examined after an effective training experience, novel CS-US pairing, and compared with an ineffective one, familiar CS-US pairing. After CS-US pairing, taste novelty modulated FLI in virtually all the regions previously implicated in conditioned taste aversion (CTA) learning, including PBN, CNA, BLA, IC, as well as NTS. Thus, a distributed and interdependent neural CTA circuit is mapped using this method, and the use of localized lesion and inactivation studies promises to further define the functional role of structures within this circuit.

WITHDRAWN: Phosphatase blockade in the amygdala enhances both conditioned taste aversion learning and cAMP-mediated cellular activation

The publisher regrets that this article is an accidental duplication of an article that has already been published, doi: 10.1016/j.appet.2006.03.001 . The duplicate article has therefore been withdrawn.

WITHDRAWN: Effects of different gustatory nuclei lesions on conditioned taste aversion learning in rats

The publisher regrets that this article is an accidental duplication of an article that has already been published, doi: 10.1016/j.appet.2006.03.001 . The duplicate article has therefore been withdrawn.

Naloxone, not proglumide or MK-801, alters effects of morphine preexposure on morphine-induced taste aversions

Both cholecystokinin (CCK) antagonists and N-methyl- d-aspartate (NMDA) antagonists block or reduce the development of morphine tolerance in several analgesic assays. The present experiments were performed to assess the ability of the CCK antagonist proglumide and the NMDA antagonist MK-801 to affect tolerance to the aversive properties of morphine as indexed by conditioned taste aversion (CTA) learning. Specifically, male Sprague–Dawley rats were exposed to either vehicle or morphine (5 mg/kg) in combination with either proglumide (5 mg/kg; Experiment 1), MK-801 (0.1 mg/kg; Experiment 2) or naloxone (1, 3.2 mg/kg; Experiment 3). Saccharin was then presented and was followed by an injection of either vehicle or morphine (10 mg/kg). Animals preexposed to and conditioned with morphine acquired an attenuated morphine-induced aversion to saccharin. While neither proglumide nor MK-801 had an effect on this attenuation, naloxone blocked the effects of morphine preexposure, suggesting that neither CCK nor NMDA may be involved in the aversive effects of morphine (or their modulation by drug exposure). That the attenuating effects of morphine preexposure on a morphine-induced CTA can be blocked suggests that the weakening of the aversive effects of morphine with chronic use can be prevented, an effect that may have implications for overall drug acceptability.

WITHDRAWN: “Asymmetric lesions of the gustatory thalamocortical loop selectively disrupt morphine-induced contrast, while sparing LiCl-induced conditioned taste aversion (CTA) learning”

The publisher regrets that this article is an accidental duplication of an article that has already been published, doi: 10.1016/j.appet.2006.03.001 . The duplicate article has therefore been withdrawn.

Serotonin and dopamine in the parabrachial nucleus of rats during conditioned taste aversion learning

A microdialysis technique was used to monitor changes in serotonin (5-HT), 5-hydroxyindole acetic acid (5-HIAA) and dopamine (DA) in the extracellular space of the parabrachial nucleus (PBN) of rats to estimate the contribution of these neurotransmitter systems to the acquisition of conditioned taste aversion (CTA). A significant (280%) enhancement of 5-HT was found immediately after saccharin drinking (CS). I.p. injection of unconditioned stimulus LiCl alone (after water drinking) also increased level of 5-HT (200%). However, when saccharin intake was followed by injection of LiCl (CS–US pairing), no change in 5-HT was observed. 5-HIAA and DA were unaffected by any of the above treatments. Thus in spite of elevation of 5-HT in PBN following saccharin consumption alone (CS) or LiCl administration alone (US) no changes in 5-HT occurred after pairing of both stimuli (CS–US). Our work demonstrates that participation of 5-HT in acquisition of CTA appears to be unlikely, and also DA appears not to be engaged in this acquisition at all. At the level of the PBN 5-HT participates mainly in CS and/or US stimuli processing, where this phenomenon has close relationship to other important physiological mechanisms, involved in behavioral control. Such as anxiety, alimentation intake.

The effects of acute corticosterone on lithium chloride-induced conditioned place aversion and locomotor activity in rats

Acute administration of corticosterone (CORT) facilitates learning in a number of associative paradigms including lithium chloride (LiCl)-induced conditioned taste aversion learning. The present study examined the effects of acute CORT on LiCl-induced conditioned place aversions in male rats. Automated open-fields were partitioned into two chambers distinct in tactile and visual cues. Animals received either LiCl (64 mg/kg, 0.15 M) or saline (NaCl, 0.15 M) followed 10 min later by either CORT (5 mg/kg) or β-cyclodextrin vehicle (45%) prior to placement in one of the chambers. Control rats received NaCl-Vehicle paired with both chambers. Three experimental groups received either NaCl-CORT, LiCl-Vehicle or LiCl-CORT paired with the preferred chamber and NaCl-Vehicle (control) paired with the non-preferred chamber. During extinction trials, animals were allowed to choose between the two chambers. Locomotor activity and its distribution within the chambers were assessed during both conditioning and extinction trials. CORT administration produced significant increases in a variety of measures of locomotor activity during conditioning trials. During extinction trials both LiCl groups displayed a conditioned place aversion while the NaCl-CORT group did not. In addition, significant increases in vertical activity were recorded in both LiCl groups in the LiCl-paired chamber. Moreover, CORT administration had no effect on LiCl-induced conditioned place aversion as time spent in the LiCl-paired chamber did not significantly differ between LiCl-Vehicle and LiCl-CORT groups. Significant increases in a number of measures of horizontal activity were also observed in both CORT groups. The present study shows that acute CORT administration does not significantly influence LiCl-induced conditioned place aversions and suggests that the facilitatory effects of acute CORT administration on learning are highly context-dependent.

Requirement of New Protein Synthesis of a Transcription Factor for Memory Consolidation: Paradoxical Changes in mRNA and Protein Levels of C/EBP

Some specific transcription factors are essential for memory consolidation across species. However, it is still unclear whether only the activation of constitutively expressed forms of these conserved transcription factors is involved in memory consolidation or their de novo synthesis also occurs after learning. This question has remained unanswered partly because of the lack of an efficient method for the determination of copy numbers of particular mRNAs in single neurons, which allows the detection of new transcription at the cellular level. Here we applied a newly developed protocol of single-cell quantitative real-time polymerase chain reaction (qRT-PCR) to single neurons playing an important role in associative learning. Specifically, we examined the changes in the mRNA and protein expression levels of a highly conserved transcription factor, CCAAT/enhancer binding protein (C/EBP), in the paired B2 motoneurons of the pond snail Lymnaea stagnalis. These buccal neurons are involved in the motor control of feeding behavior, with a potentially important role in conditioned taste aversion (CTA). Single-cell qRT-PCR revealed a significant decrease in LymC/EBP mRNA copy numbers in the B2 motoneurons during memory consolidation after CTA training. By contrast, isoelectric focusing and immunoblotting of extracts of the buccal ganglia showed that translation and phosphorylation levels of LymC/EBP significantly increased during memory consolidation. The C/EBP-like immunoreactivity in the B2 motoneurons, which are the major immunopositive component in the buccal ganglia, also significantly increased during memory consolidation, suggesting that the main source of increase in the level of protein in the buccal ganglia are the B2 motoneurons. Thus, early memory consolidation after CTA learning in L.stagnalis involves both the rapid synthesis and phosphorylation of LymC/EBP as well as the rapid breakdown of LymC/EBP mRNA in the neural network controlling feeding, suggesting that all of these processes play a role in the function of C/EBP in memory consolidation.

Hippocampal inactivation enhances taste learning

Learning tasks are typically thought to be either hippocampal-dependent (impaired by hippocampal lesions) or hippocampal-independent (indifferent to hippocampal lesions). Here, we show that conditioned taste aversion (CTA) learning fits into neither of these categories. Rats were trained to avoid two taste stimuli, one novel and one familiar. Muscimol infused through surgically implanted intracranial cannulae temporarily inactivated the dorsal hippocampus during familiarization, subsequent CTA training, or both. As shown previously, hippocampal inactivation during familiarization enhanced the effect of that familiarization on learning (i.e., hippocampal inactivation enhanced latent inhibition of CTA); more novel and surprising, however, was the finding that hippocampal inactivation during training sessions strongly enhanced CTA learning itself. These phenomena were not caused by specific aspects of our infusion technique--muscimol infusions into the hippocampus during familiarization sessions did not cause CTAs, muscimol infusions into gustatory cortex caused the expected attenuation of CTA, and hippocampal inactivation caused the expected attenuation of spatial learning. Thus, we suggest that hippocampal memory processes interfere with the specific learning mechanisms underlying CTA, and more generally that multiple memory systems do not operate independently.

The effects of cocaine, alcohol and cocaine/alcohol combinations in conditioned taste aversion learning

We have recently reported that alcohol attenuates cocaine place preferences. Although the basis for this effect is unknown, alcohol may attenuate cocaine reward by potentiating its aversive effects. To examine this possibility, these experiments assessed the effects of alcohol on cocaine-induced taste aversions under conditions similar to those that resulted in attenuated place preferences. Specifically, Experiments 1 and 2 assessed the effects of alcohol (0.5 g/kg) on taste aversions induced by 20, 30 and 40 mg/kg cocaine. Experiment 3 examined the role of intertrial interval in the effects of alcohol (0.5 g/kg) on cocaine (30 mg/kg) taste aversions. In Experiments 1 and 2, cocaine was effective at conditioning aversions. Alcohol produced no measurable effect. Combining cocaine and alcohol produced no greater aversion than cocaine alone (and, in fact, weakened aversions at the lowest dose of cocaine). In Experiment 3, varying the intertrial interval from 3 days (as in the case of Experiments 1 and 2) to 1 day (a procedure identical to that in which alcohol attenuated cocaine place preferences) resulted in significant alcohol- and cocaine-induced taste aversions. Nonetheless, alcohol remained ineffective in potentiating cocaine aversions. Thus, under these conditions alcohol does not potentiate cocaine's aversiveness. These results were discussed in terms of their implication for the effects of alcohol on cocaine-induced place preferences. Further, the effects of alcohol on place preferences conditioned by cocaine were discussed in relation to other assessments of the effects of alcohol on the affective properties of cocaine and the implications of these interactions for alcohol and cocaine co-use.

Nω-nitro-l-arginine methyl ester attenuates lithium-induced c-Fos, but not conditioned taste aversion, in rats

Lithium chloride (LiCl) at doses sufficient to induce conditioned taste aversion (CTA) causes c-Fos expression in the relevant brain regions and activates the hypothalamic-pituitary-adrenal (HPA) axis. It has been suggested that nitric oxide (NO) in the central nervous system may play a role not only in the activation of HPA axis but also in CTA learning, and that LiCl may activate the brain NO system. To determine the role of NO in lithium-induced CTA, we examined the lithium-induced CTA, brain c-Fos expression, and plasma corticosterone level with Nomega-nitro-L-arginine methyl ester (L-NAME) pretreatment. Intraperitoneal L-NAME (30 mg/kg) given 30 min prior to LiCl significantly decreased lithium-induced c-Fos expression in the brain regions implicated in CTA learning, such as the hypothalamic paraventricular nucleus (PVN), central nucleus of amygdala (CeA), and nucleus tractus of solitarius. However, either the lithium-induced CTA acquisition or the increase in plasma corticosterone was not attenuated by l-NAME pretreatment. These results suggest that NO may be involved in lithium-induced neuronal activation of the brain regions, but not in the CTA acquisition or the HPA axis activation.

Impaired conditioned taste aversion learning in APP transgenic mice

Cognition in transgenic mouse models of Alzheimer’s disease (AD) has been predominantly characterized in explicit spatial orientation tasks. However, dementia in AD encompasses also implicit memory systems. In the present study a line of transgenic mice (TgCRND8) encoding a double mutated allele of the human amyloid precursor protein ( APP) genes was evaluated in an implicit associative learning task of conditioned taste aversion (CTA). CTA is a form of Pavlovian classical conditioning, in which a mouse learns to avoid a novel taste of saccharine (conditioned stimulus) paired with an experimentally induced (systemic injection of lithium chloride) nausea (unconditioned stimulus). In contrast to conditioned non-Tg mice, TgCRND8 APP mice developed weaker aversion against saccharine and quickly increased its consumption in repeated tests. These results indicate that TgCRND8 mice show a significant impairment not only in explicit spatial memory, as has been previously shown [Nature 408 (2000) 979], but also in implicit memory. Control experiments confirmed that TgCRND8 and non-Tg mice had comparable taste sensitivities in response to appetitive as well as aversive tastes. The study suggests that the CTA paradigm can be a sensitive tool to evaluate deficits in implicit associative learning in APP transgenic mouse models of AD.

Conditioned taste aversion: a database

The present commentary describes the availability of a database on conditioned taste aversion learning, the avoidance of fluids and foods previously associated with the aversive effects of a variety of drugs. The database includes articles as early as 1955 by Garcia and his colleagues [Science 122 (1955) 127] (reporting such avoidance in rats) and papers just published given that the database is ongoing and constantly updated. At the printing of this announcement, approximately 2600 papers are included in the database. The database allows the user to search for articles by author(s), key word(s), date, title and journal. These terms can be used as single entries or multiple combinations. Finally, the full database can be viewed by performing the search function without entering any terms in the query window. The database can be accessed at http://www.CTALearning.com.

The effects of proton exposure on neurochemistry and behavior

Future space missions will involve long-term travel beyond the magnetic field of the Earth, where astronauts will be exposed to radiation hazards such as those that arise from galactic cosmic rays. Galactic cosmic rays are composed of protons, α particles, and particles of high energy and charge (HZE particles). Research by our group has shown that exposure to HZE particles, primarily 600 MeV/n and 1 GeV/n 56Fe, can produce significant alterations in brain neurochemistry and behavior. However, given that protons can make up a significant portion of the radiation spectrum, it is important to study their effects on neural functioning and on related performance. Therefore, these studies examined the effects of exposure to proton irradiation on neurochemical and behavioral endpoints, including dopaminergic functioning, amphetamine-induced conditioned taste aversion learning, and spatial learning and memory as measured by the Morris water maze. Male Sprague–Dawley rats received a dose of 0, 1.5, 3.0 or 4.0 Gy of 250 MeV protons at Loma Linda University and were tested in the different behavioral tests at various times following exposure. Results showed that there was no effect of proton irradiation at any dose on any of the endpoints measured. Therefore, there is a contrast between the insignificant effects of high dose proton exposure and the dramatic effectiveness of low dose (<0.1 Gy) exposures to 56Fe particles on both neurochemical and behavioral endpoints.

Morphine- and cocaine-induced c-Fos levels in Lewis and Fischer rat strains

Lewis (LEW) and Fischer 344 (F344) rat strains have been reported to differ in their sensitivity to the rewarding and aversive efffects of both cocaine and morphine. Specifically, LEW rats self-administer morphine and cocaine to a greater extent than F344 rats, while LEW (compared to F344) rats are more sensitive to the aversive effects of cocaine but less sensitive to the aversive effects of morphine. Consistent with assessments of the rewarding effects of morphine and cocaine in these two strains, LEW rats have lower basal, and generally higher drug-induced, activity in brain regions associated with reward. Although the brain areas that mediate the aversive effects of drugs are becoming better defined, no studies have compared the activation of these areas by aversion-inducing drugs in the LEW and F344 strains. As such, the relationship between the ability of drugs to activate these aversion-associated brain areas and to induce a conditioned taste aversion (CTA) in these strains is unknown. To explore this relationship, LEW and F344 rats were injected with saline or doses of morphine or cocaine (32 mg/kg for both drugs) that have been shown to generate differential taste aversion learning in these strains. All animals were subsequently tested for c-Fos expression in areas of the brain associated with aversion learning (the lateral and medial parabrachial nucleus, intermediate and caudal nucleus tractus solitarius and area postrema), reward (the shell of the nucleus accumbens) and locomotion (the core of the nucleus accumbens and the caudate putamen). The present results indicated that patterns of morphine- and cocaine-induced c-Fos within CTA-associated, but not reward- or locomotor-associated, brain regions paralleled the differential behavioral sensitivities of LEW and F344 rats to these drugs within CTA learning. Analyses with other drugs that do and do not induce aversions differentially would further assess the role of these brain areas in aversion learning, in general, and in strain-dependent differences, in particular.

The CS-Preexposure Effect in Conditioned Taste-Aversion Learning in Golden Hamsters

In Experiment 1, golden hamsters were injected with either 0.9% saline or the nausea-inducing agent, lithium chloride (LiCL), immediately after consuming a flavored diet that was either novel or familiar. The LiCl-induced aversion was strong in hamsters for which the flavored diet was novel, but no significant aversion was observed in hamsters that were familiar with the flavored diet. In Experiment 2, the strength of the LiCl-induced aversion was related inversely to the amount of conditioned-stimulus (CS) preexposure and directly to the duration of the preexposure-conditioning interval. Thus, although some previous researchers have suggested that hamsters may not demonstrate the CS-preexposure effect in a conditioned taste-aversion paradigm, they clearly did so under the conditions of the present experiments, and moreover, the characteristics of the CS-preexposure effect in hamsters were generally similar to those observed in rats.

Facilitation of conditioned taste aversion learning by systemic amphetamine: role of nucleus accumbens shell dopamine D1 receptors

The role of dopamine (DA) in associative learning was studied in a conditioned taste aversion (CTA) paradigm with sucrose as the conditioned stimulus (CS) and intraperitoneal lithium chloride as the unconditioned stimulus (US). Drinking on trial of a 15% sucrose solution followed 1 h later by lithium chloride (20 or 40 mg/kg i.p.) resulted in mild CTA, as shown by reduction of drinking of the sucrose solution 24 h later. Amphetamine sulphate (0.125, 0.25, 0.50 and 1.0 mg/kg s.c.), administered on trial 5 min after sucrose drinking, facilitated CTA with maximal effects at 0.25 mg/kg s.c. Amphetamine given in the absence of lithium or 45 min after sucrose did not affect sucrose intake. The DA D1 receptor antagonist SCH 39166, administered before amphetamine either systemically (0.0125 mg/kg s.c.) or in the nucleus accumbens shell (NAc; 0.025 micro g/ micro L on each side) prevented the facilitation of CTA induced by amphetamine. It is concluded that amphetamine facilitates CTA learning by strengthening the consolidation of gustatory short-term memory via D1 receptors of the NAc shell.

Does cAMP Response Element-Binding Protein Have a Pivotal Role in Hippocampal Synaptic Plasticity and Hippocampus-Dependent Memory?

Previous studies addressing the role of the transcription factor cAMP response element-binding protein (CREB) in mammalian long-term synaptic plasticity and memory by gene targeting were compromised by incomplete deletion of the CREB isoforms. Therefore, we generated conditional knock-out strains with a marked reduction or complete deletion of all CREB isoforms in the hippocampus. In these strains, no deficits could be detected in lasting forms of hippocampal long-term potentiation (LTP) and long-term depression (LTD). When tested for hippocampus-dependent learning, mutants showed normal context-dependent fear conditioning. Water maze learning was impaired during the early stages, but many mutants showed satisfactory scores in probe trials thought to measure hippocampus-dependent spatial memory. However, conditioned taste aversion learning, a putatively hippocampus-independent memory test, was markedly impaired. Our data indicate that in the adult mouse brain, loss of CREB neither prevents learning nor substantially affects performance in some hippocampus-dependent tasks. Furthermore, it spares LTP and LTD in paradigms that are sensitive enough to detect deficits in other mutants. This implies either a species-specific or regionally restricted role of CREB in the brain and/or a compensatory upregulation of the cAMP response element modulator (CREM) and other as yet unidentified transcription factors.

Reversal of long-delay conditioned taste aversion learning in rats by sex hormone manipulation.

Conditioned taste aversion (CTA) learning is an adaptive, robust, well-established learning and memory paradigm. Strong taste aversions develop to the conditioned stimulus (CS = saccharin) despite long delays between exposure to the CS and unconditioned stimulus (US = LiCl). Rats display a sexually dimorphic pattern of long-delay CTA learning (Foy et al., 1996). The present study examines whether this sex difference is a result of activational or organizational hormone action, because here we implanted gonadectomized rats with their normal hormone replacements, or with opposing hormones prior to testing in a 4-hr delayed CTA learning task. We found that gonadally intact male rats displayed a more robust CTA response than intact female rats. Gonadectomy essentially eliminated this sex difference; gonadectomized males and gonadectomized females displayed similar CTA responses. In gonadectomized rats, when their normal sex hormones were replaced with implanted hormone pellets, the sex difference in CTA learning was reinstated. In contrast, when gonadectomized rats were implanted with opposing hormones, the sex difference was reversed. Gonadectomized female rats implanted with 5alpha-DHT pellets (metabolite of testosterone) displayed a stronger CTA response compared to gonadectomized males implanted with 17beta-estradiol pellets. Regardless of the original developmental hormonal environment, our study suggests that an activational manipulation of circulating hormones serves to significantly influence long-delay CTA learning in rats.

Conditioned taste aversion learning in leptin-receptor-deficient db/ db mice

The db/ db mouse has defective leptin receptors. The defects lead to impairments of leptin regulation of food intake and body weight, and result in the expression of diabetic symptoms such as hyperinsulinemia, hyperglicemia, and extreme obesity. Recent studies have proposed that leptin may also affect memory and learning processes. To examine this possibility, we compared the ability of leptin-receptor-deficient db/ db mice and their normal lean litter mates to form and extinguish a conditioned taste aversion (CTA) for saccharin. We used a short-term (10 s) lick test and a long-term (48 h) two bottle preference test for measurement of consumption of test solutions. On the first day after conditioning to avoid saccharin, the db/ db mice showed preference scores for saccharin as low, and aversion thresholds for sucrose lower than that of the lean mice. During the extinction test trials beginning from the second up to the 30th day after conditioning, numbers of licks and preference scores for aversive saccharin and sucrose appeared to be larger, and recovered faster to the control levels in db/ db mice. These results indicate that db/ db mice with leptin-receptor-deficiency may show equal capacity to form CTAs for saccharin, greater generalization from saccharin to sucrose, and a faster rate of extinction. This suggests that disruption of leptin signalling does not inhibit acquisition of CTA learning, but impairs its extinction. This differential contribution of the leptin system on CTA processes may be due to differential distribution of leptin receptors in the CTA-related brain areas.