Cocaine-induced conditioned taste aversions in male and female Wistar rats

After an initial period of adaptation to 20 min per day of limited water availability, male and female Wistar rats were allowed access to water or a 0.1% sodium-saccharin solution. Saccharin exposures were followed by the subcutaneous administration of 0, 5, 10 or 20 mg/kg cocaine for different groups of rats. Four pairings of the saccharin solution with cocaine administration resulted in a consistent decrease in saccharin consumption only in female subjects injected with the largest dose of cocaine (20 mg/kg). Choice testing in which subjects could choose between two drinking tubes, one containing water, the other one containing the saccharin solution, was then conducted during extinction. During four of such experimental sessions, subjects which had previously been injected with vehicle mostly consumed the saccharin solution or showed a position bias. Conditioned taste aversions were not only observed in the group of female subjects injected with 20 mg/kg cocaine, but also in males previously treated with 20 mg/kg cocaine. In addition, compared to vehicle control groups, males and females injected with 5 and 10 mg/kg cocaine tended to avoid the saccharin solution in favor of regular water. It is suggested that previous failures to obtain consistent cocaine-mediated taste aversions may have been a function of the experimental procedures used to assess cocaine's efficacy in inducing conditioned taste aversions.     

Asymmetric serial interactions between ethanol and cocaine in taste aversion learning

Although the interaction between ethanol and cocaine is well documented, it has generally been limited to situations in which the two drugs are given concurrently. Little exists on the interaction between ethanol and cocaine when one drug is given prior to the other. In Experiment 1, female Long-Evans rats were given five exposures to ethanol (2 g/kg ip) or vehicle prior to taste aversion conditioning with cocaine (32 mg/kg sc) for a total of five conditioning trials. In Experiment 2, rats were given five exposures to cocaine (32 mg/kg sc) or vehicle prior to taste aversion conditioning with ethanol (2 g/kg ip) for a total of five conditioning trials. Ethanol-preexposed, cocaine-conditioned animals (Experiment 1) displayed attenuated aversions to the cocaine-associated solution, drinking significantly greater amounts of saccharin than vehicle-preexposed, conditioned subjects. Conversely, cocaine-preexposed, ethanol-conditioned animals (Experiment 2) displayed robust aversions to the ethanol-associated solution, drinking levels comparable to those consumed by vehicle-preexposed, conditioned subjects and drinking significantly less than controls. Although the basis for these asymmetric effects is not known, they may have implications for abuse vulnerability in that drug history may impact subsequent drug toxicity that, in turn, may alter drug acceptability.     

Antagonism of drug discrimination learning within the conditioned taste aversion procedure.

Animals injected with morphine prior to the presentation of a saccharin-LiCl pairing and the morphine vehicle prior to saccharin alone rapidly acquired the drug discrimination, avoiding saccharin following the administration of morphine and consuming saccharin following its vehicle after only four conditioning trials. Once stimulus control was established, the opiate antagonist naloxone (1 mg/kg) was administered prior to morphine in a test of its ability to antagonize the morphine stimulus. Pretreatment times ranged from 10 to 180 min. Naloxone antagonized the stimulus properties of morphine for all subjects, although there were individual differences in the onset, duration (time course) and degree of antagonism. Together with the rapid acquisition typically reported in this design, the fact that antagonism was demonstrated in the present study suggests that the conditioned taste aversion procedure may be useful in the general assessment of drug discriminations.     

Cocaine-induced conditioned taste aversions in rats

In two separate studies cocaine hydrochloride at doses between 10--36 mg/kg was found to induce a dose-related conditioned taste aversion (C.T.A.) to saccharin, and to be an effective conditioning agent even when injections of the drug were delayed 90 min after saccharin intake. These data contrast with conditioning agent when unjectuons of the drug were delayed 90 min after saccharin intake. These data contrast with an earlier report [3] which suggested that cocaine was totally devoid of aversive properties. However, they do indicate that cocaine is only a weak aversion-inducing agent. In contrast to other drugs, the doses of cocaine which are required to induce a C.T.A. are very large relative to those commonly employed in behavioural studies. The weak potency of cocaine in inducing C.T.A. may be related to the drug's marked potency in the self-administration paradigm. Some possible determinants of cocaine's weak effects are discussed.     

D-cycloserine enhances short-delay, but not long-delay, conditioned taste aversion learning in rats

NMDA receptors have been implicated in conditioned taste aversion (CTA), a form of associative learning with the unique temporal characteristic of associating taste and toxic stimuli across very long delays. d-cycloserine (DCS), an NMDA receptor agonist, has been shown to enhance short-delay CTA learning. Here we examined the interaction of DCS with varying temporal parameters of CTA. DCS (15 mg/kg) administered prior to the pairing of 0.125% saccharin and LiCl (38 mM, 12 ml/kg) enhanced CTA when there was a short delay between the taste-toxin pairing (10 min), but not when there was a long delay (45 min). DCS activity remained at effective levels over the long delay, because DCS administered 60 min prior to a short-delay pairing enhanced CTA. The interaction of DCS with the delay between taste stimulus onset and LiCl injection was investigated by administering DCS and then 5 min access to saccharin 45 min prior to a short-delay pairing of saccharin and LiCl. DCS failed to enhance CTA in rats pre-exposed to saccharin, even with a short delay between the second saccharin exposure and LiCl injection. These results suggest that DCS enhancement of CTA is dependent on mechanisms underlying gustatory processing during long-delay taste-toxin associations.     

Ethanol preexposure attenuates the interaction of ethanol and cocaine in taste aversion learning

Although the potentiating effects of ethanol and cocaine have been well documented, little has been reported regarding the effects of ethanol or cocaine history on this interaction. In the present study, female Long-Evans rats received five exposures to ethanol (3.5 g/kg ip) or vehicle prior to taste aversion conditioning in which a novel saccharin solution was paired with either ethanol (0.56 g/kg ip), cocaine (25 mg/kg sc) or the combination (or the drugs' vehicle) for a total of five conditioning trials. Nonpreexposed subjects conditioned with the ethanol/cocaine combination displayed aversions, drinking levels significantly less than nonpreexposed subjects conditioned with either drug alone. Further, the aversions produced by the combination were greater than the sum of the aversions produced by ethanol and cocaine, alone. Ethanol-preexposed animals conditioned with the combination displayed an attenuated aversion, drinking significantly greater amounts of saccharin than nonpreexposed conditioned subjects and not differing from controls. Although the basis for the attenuation by ethanol of the aversions induced by the drug combination is not known, the present findings may have implications for the use and abuse of the combination in that alcohol history may reduce the subsequent toxicity of the combination that in turn may affect its acceptability.     

Acetaldehyde may mediate reinforcement and aversion produced by ethanol. An examination using a conditioned taste-aversion paradigm

Groups of water-deprived rats were exposed to acetaldehyde, ethanol or vehicle control. On the conditioning day, the animals were first presented with a solution of saccharin after which the animals that were exposed to acetaldehyde received ethanol and those exposed to ethanol received acetaldehyde. Saccharin was again presented on three more occasions (testing days) without injection of drug. Using the percentage change in saccharin consumed from the first presentation as a measure of aversion, it was found that exposure to acetaldehyde blocked the taste aversion conditioned by ethanol. Animals exposed to ethanol showed no aversion to the saccharin which was paired with a small dose of acetaldehyde, indicating a symmetrical relationship between ethanol and acetaldehyde at this dose. However, exposure with ethanol did not block the aversion produced by conditioning with larger doses of acetaldehyde. These results suggest that the mechanism underlying the smaller dose of the taste aversion conditioned with acetaldehyde may be central while the mechanism underlying the larger dose is probably peripheral.     

Nicotine-induced conditioned taste aversion in the rat: effects of ethanol

It has been shown that small doses of ethanol antagonise the discriminative stimulus properties of nicotine in the rat. The aim of the present study was to evaluate whether ethanol could antagonise the aversive stimulus effects of nicotine. Wistar rats were trained to associate nicotine injections with a novel tasting fluid (0.1% saccharin) in the conditioned taste aversion procedure. Nicotine (0.3 mg/kg, s.c.) was injected 5 min after the end of a 20-min exposure to the saccharin solution. Ethanol (0.25-0.5 g/kg, i.p.) was administered 5 or 50 min before nicotine. In general, ethanol did not inhibit nicotine-induced conditioned taste aversion. Contrary to the findings in drug discrimination studies, a slight but significant enhancement of nicotine-induced taste aversion conditioning was observed after ethanol pre-treatment. Blood ethanol levels were measured in a separate group of rats. Maximal blood ethanol levels after i.p. administration of 0.25 or 0.5 g/kg ethanol exceeded 20 and 80 mg%, respectively. Concluding, the present results may indicate that ethanol does not attenuate nicotine-induced conditioned taste aversion in the rat.     

Morphine conditioned taste aversion revered by periaqueductal gray lesions

The role of the periaqueductal gray (PAG) in morphine conditioned taste aversion (CTA) was studied using male Wistar rats as subjects. Following the presentattion of a novel saccharin solution, animals with or without a lesion of the PAG were intraperitoneally injected with either morphine, lithium, ethanol or fenfluramine. As evident by the amount of saccharin of saccharin solution consumed on a subsequent presentation, a PAG lesion reversed a morphine CTA but not CTAs produced by the other drugs used. The results suggest that the PAG may in part mediate morphine CTA.     

Effect of sodium deprivation on morphine-and lithium-induced conditioned salt avoidance and taste reactivity

RATIONALE: When paired with morphine, rats suppress their intake of saccharin solution, but not a less palatable salt solution. The reward comparison hypothesis argues that when a taste is paired with morphine, intake of the solution is expected to decrease as the palatability of the taste increases. Therefore, morphine should more effectively suppress intake of salt solution in rats that are conditioned in a sodium-deprived state than in rats that are conditioned in a sodium-replete state. OBJECTIVES: The present experiments evaluated the effect of furosemide-induced sodium deprivation on morphine and lithium-induced salt (experiment 1) and saccharin (experiment 2) avoidance and salt taste reactivity (experiment 4). METHODS: Rats were injected with furosemide or saline 21 h prior to access to salt solution (experiments 1 and 3) or saccharin solution (experiment 2). Immediately following access to the solution, the rats were injected with saline, morphine or lithium chloride solution. In experiments 1 and 2, a two-bottle test measured the strength of the taste preference/avoidance. In experiments 3 and 4, the taste reactivity test evaluated the furosemide-induced unconditional palatability changes for salt solution (experiment 3) and the conditional palatability changes for salt previously paired with morphine or lithium (experiment 4). RESULTS: Sodium depletion induced by furosemide pretreatment conditionally enhanced subsequent preference for salt solution using both the taste avoidance test (experiment 1) and the taste reactivity test (experiment 4). Salt-lithium associations, but not salt-morphine associations, suppressed salt preference. However, the salt-morphine (40 mg/kg) association enhanced salt preference (in both experiments 1 and 4) when rats were conditioned in a sodium-deprived state. In experiment 2, morphine-saccharin associations resulted in conditioned saccharin avoidance regardless of pretreatment condition. CONCLUSIONS: When the palatability of salt was enhanced by sodium depletion, morphine produced a mild conditioned salt preference in both a two-bottle preference test and enhanced ingestion reactions in the taste reactivity test, but morphine produced conditioned saccharin avoidance.     

Reducing endocannabinoid metabolism with the fatty acid amide hydrolase inhibitor, URB597, fails to modify reinstatement of morphine-induced conditioned floor preference and naloxone-precipitated morphine withdrawal-induced conditioned floor avoidance

The potential of the fatty acid amide hydrolase (FAAH) inhibitor, URB597, to modify drug prime-induced reinstatement of morphine-induced conditioned floor preference or naloxone-precipitated morphine withdrawal-induced conditioned floor avoidance was evaluated. In Experiment 1, morphine-induced conditioned floor preference was established across 4 conditioning trials. Following extinction training (4 trials), rats were pretreated with URB597 or vehicle prior to a morphine prime or a saline prime. Morphine reinstated the previously extinguished floor preference, but URB597 did not modify the strength of the reinstated preference. In Experiment 2, naloxone-precipitated morphine withdrawal-induced conditioned floor avoidance was established across 2 conditioning trials. Following extinction training (14 trials), rats were pretreated with URB597 or vehicle prior to a saline prime or a morphine withdrawal prime. The morphine withdrawal prime reinstated the previously extinguished floor avoidance, but URB597 did not modify the strength of reinstated avoidance. These results suggest that under the conditions in which URB597 promotes extinction (e.g., Manwell et al. (2009)) it does not interfere with drug-induced reinstatement of either conditioned floor preference or avoidance. That is, although activation of the endocannabinoid (eCB) system promotes extinction of aversive learning, it may not prevent reinstatement of that aversion by re-exposure to the aversive treatment.     

Morphine-induced conditioned taste aversions: assessment of sexual dimorphism

Although sex differences in taste aversions have been reported with emetics such as lithium chloride (LiCl), little is known whether such findings generalize to other aversion-inducing drugs, including recreational compounds. One particular class of recreational compounds that induces taste aversions but that has not been examined for sex differences in its aversive properties is the opioids. To assess sex differences in the aversive properties of the opioids, Experiment 1 examined the acquisition and extinction of morphine-induced taste aversions in male and female rats. To determine whether the specific parametric conditions used in Experiment 1 would support sex differences in general, Experiment 2 examined possible sex differences in the acquisition and extinction of LiCl-induced taste aversions, a compound for which sex differences have been previously reported. During acquisition, male and female rats were given 20-min access to a novel saccharin solution and injected with either morphine (0, 10, 18 and 32 mg/kg s.c.; Experiment 1) or LiCl (0, 0.3, 0.6 and 1.2 mEq s.c.; Experiment 2) every fourth day for a total of four conditioning trials. During extinction, subjects were allowed access to saccharin but were not injected (for a total of eight trials). There were no sex differences in acquisition with either morphine or LiCl. There were also no sex differences in extinction with morphine; however, sex differences were found with LiCl, an effect consistent with prior assessments with this drug. The basis for and implications of the differences in the effects of sex on morphine- and LiCl-induced taste aversions were discussed.     

Discriminated conditioned taste aversion for studying multi-element stimulus control

The effects of morphine pre-treatment interval on the stimulus control exerted by a multi-element stimulus consisting of morphine (5.6mg/kg), saccharin (0.2%, w/v), and a ball-bearing drinking nozzle in a discriminated taste aversion procedure were examined. In this discriminated aversion procedure, rats received injections of LiCI following presentation of this multielement stimulus, and injections of saline following the saline, water, and non-ball-bearing nozzle composite stimulus. These paired rats were compared to unpaired rats that received saline injections rather than LiCI injections following presentation of the multi-element stimulus. Morphine pre-treatment times of 5, 10, and 20min were examined in groups of 12 paired and 6 unpaired rats. The discrimination was rapidly learned under all three pre-treatment intervals. In subsequent testing with each individual stimulus element and combinations of two stimulus elements, stimulus control was clearly exerted by both morphine and saccharin. Paired rats drank less saccharin than unpaired rats, and less saccharin than water. Similarly, paired rats drank less fluid following morphine administration than following saline administration, and less fluid than unpaired rats following morphine administration. Control by the nozzle type was also apparent in significant interactions between the nozzle and morphine or saccharin and pairing with LiCI. In general, pre-treatment time did not influence the stimulus control that developed. However, at the two shorter pre-treatment times there was some indication that a conditioned taste aversion to morphine was developing in the unpaired rats. These experiments indicate that such discriminated taste aversion procedures may be viable methods for studying the contextual control of how drugs function as discriminative stimuli, and that longer drug pre-treatment times may be desirable in such procedures.     

Assessing learned associations between conditioned cocaine reward and environmental stimuli in the Wistar Kyoto rat

Clinical studies demonstrate that anxiety disorders increase the risk of substance use disorder. However, few studies have directly assessed anxiety as a vulnerability factor in processing of rewarding stimuli. The Wistar-Kyoto (WKY) rat has been proposed as a model of anxiety vulnerability because it exhibits extreme behavioral inhibition in novel and social environments; yet, it displays paradoxical rapid active avoidance learning that is resistant to extinction. The present study was designed to characterize the acquisition and persistence of cocaine conditioned place preference (CPP) in WKY rats. In the first of a series of three experiments, adult male WKY and Sprague Dawley (SD) rats were given six pairings of cocaine (3, 5, 10, 15 mg/kg) or saline on alternating days. SD rats developed cocaine-induced CPP to each of the four doses of cocaine tested. In contrast, WKY rats demonstrated CPP when conditioned with 3, 5, and 10 mg/kg, but displayed no preference to the 15 mg/kg dose. Next, separate groups of rats were subject to an extended CPP paradigm, which included acquisition, extinction and reinstatement phases. Rats were conditioned with cocaine and saline on alternating days using either a 6/6 (as above) or 4/4 conditioning regimen. Both SD and WKY rats acquired a lasting CPP with the 6/6 conditioning regimen. Results from the 4/4 conditioning regimen show that SD, but not WKY, rats acquired CPP. Preference scores for SD rats during the cocaine primed reinstatement test were significantly different from pretest scores indicating reinstatement of CPP in this group. Paradoxically, WKY rats demonstrated a latent sensitization to the conditioned rewarding effects of cocaine during the drug-primed reinstatement test. Taken together, WKY rats appear to be more sensitive to high doses of cocaine and need more experience with the drug to acquire a preference than SD rats.     

Strain-dependent sex differences in the effects of alcohol on cocaine-induced taste aversions

Research using the conditioned taste aversion procedure has reported that a cocaine/alcohol combination induces a significantly stronger taste aversion than either cocaine or alcohol alone. These findings suggest that the co-administration of alcohol intensifies the aversive effects of cocaine. Although the behavioral interaction of cocaine and alcohol is well established, little is known about how the effects of this drug combination might be modulated by a variety of subject variables. The current investigation addressed this by assessing if the ability of alcohol to potentiate cocaine-induced taste aversions is dependent upon the strain and/or sex of the subject. In this series of studies, male and female rats of Long-Evans (Experiment 1) and Sprague-Dawley (Experiment 2) descent were given limited access to a novel saccharin solution to drink and were then injected with either vehicle, cocaine (20 mg/kg), alcohol (0.56 g/kg) or the alcohol/cocaine combination. This procedure was repeated every fourth day for a total of four conditioning trials. All subjects were then compared on an Aversion Test that followed the fourth conditioning cycle. In three of the groups tested (male Long-Evans; male and female Sprague-Dawley), cocaine induced a significant taste aversion that was unaffected by the co-administration of alcohol. However, in female Long-Evans subjects, the addition of alcohol significantly strengthened the avoidance of the saccharin solution. Although the effects of alcohol on cocaine-induced taste aversions are dependent upon an interaction of sex and strain, the basis for this SexxStrain interaction is not known. That such an interaction is evident suggests that attention to such factors in assessing the effects of drug combinations is important to understanding the likelihood of the use and abuse of such drugs.     

Prenatal exposure to cocaine disrupts cocaine-induced conditioned place preference in rats.

Cocaine-induced conditioned place preference (CPP) was tested in adult offspring of Sprague-Dawley dams that had been injected subcutaneously with 40 mg/kg/3cc cocaine HCl (C40) daily from gestational days 8-20, pair-fed (PF) dams injected with saline, and nontreated control (LC) dams. C40 and PF dams gained significantly less weight than LC dams, although offspring body weights did not differ among the three prenatal treatment groups at birth or in adulthood. Significant place conditioning was obtained in LC and PF offspring when either 2.0 or 5.0 mg/kg of cocaine was paired with the designated place. In contrast, C40 offspring did not exhibit place conditioning at either training dose. Yet, all animals exposed to 5 mg/kg of cocaine during conditioning exhibited less activity during the test (when no cocaine was given) than controls given unpaired exposures to the apparatus and cocaine and C40 offspring did not differ from LC and PF offspring in this respect. Therefore, despite their lack of a conditioned place preference for cocaine, rats that had been exposed gestationally to cocaine nevertheless revealed an effect of cocaine during conditioning in one aspect of their test behavior. Possible explanations for the lack of cocaine-induced place preference in these animals include a learning deficit or a change in cocaine's effectiveness as a reward.     

The attenuation of a specific cue-to-consequence association by antiemetic agents

Previous research has shown that rats develop a conditioned taste aversion after a single pairing of a distinct taste and subsequent toxicosis. The experiments reported here test the hypothesis that the expression of a taste aversion may reflect classically conditioned nausea mediated by activation of brainstem emetic centers by taste stimuli. Rats were allowed to drink a saccharin solution (1 g/l) and 10 min later were intubated with LiCl (180 mg/kg) to produce nausea. When control rats were posttested for saccharin preference they consumed less than 50% of their pretest intake. Experimental rats were injected with one of four pharmacologically distinct antiemetic drugs 30 min prior to their posttest with saccharin. Each drug significantly attenuated the aversion to saccharin at one dose level. The antiemetic drugs we used were scopolamine HBr, cyclizine, prochlorperazine dimaleate, and trimethobenzamide. These drugs had no effect on the conditioned fear of a noise that signaled foot shock or on a natural aversion to a bitter fluid (quinine monohydrochloride, 100 mg/l). Our data suggest that pharmacological suppression of the neural mechanisms of emesis selectively disrupts conditioned taste aversions, and that moderate dose levels are critical for obtaining this effect.     

Both conditioned taste preference and aversion induced by corticotropin-releasing factor

Postprandial administration in the rat of a wide variety of drugs, peptides and toxins suppresses future consumption of a meal of previously unfamiliar but otherwise attractive saccharin-flavored solution. Since the intensity of this conditioned flavor aversion in the rat is sensitive to plasma stress hormone levels, the present study examined the effects on flavor conditioning of corticotropin-releasing factor, a peptide known to be involved in behavioral and hormonal responses to stress. In two-bottle water vs. saccharin choice tests, CRF (0.5 μg ICV) increased significantly the consumption of saccharin solution following a single saccharin/CRF pairing, while a tenfold larger dose of CRF (5 μg ICV) abolished saccharin intake following two saccharin/CRF pairings. Hence, exogenous CRF is capable of inducing both flavor preference and aversion in a dose- and situation-dependent manner. Further, direct neurotropic actions of CRF probably subserve its aversive effect since dexamethasone pretreatment weakened but did not prevent CRF-induced conditioned taste avoidance. These results suggest that at low doses CRF can produce arousal actions that result in taste preference and at higher doses produces aversive effects that are reflected in taste avoidance.     

Learned taste aversion to saccharin produced by orally consumed d-amphetamine

Rats were presented with solutions containing both saccharin and d-amphetamine and the development of taste aversions to solutions of either or both of these substances was studied. In Experiment 1 it was found that taste aversions developed to solutions of saccharin (1 mg/ml) which contained amphetamine at concentrations of 0.01, 0.03 and 0.1 mg/ml. Experiment 2 showed that a taste aversion conditioned to a solution of saccharin (2 mg/ml) and amphetamine (0.2 mg/ml) generalised to solutions containing saccharin at concentrations between 0.625 and 20 mg/ml but not to a solution containing only amphetamine. In the third experiment it was found that the degree of generalisation of a taste aversion to lower saccharin concentrations depended upon the concentration used during conditioning trials. When the conditioning concentration was 0.625 mg/ml the aversion generalised to concentrations as low as 0.075 mg/ml but when a 10 mg/ml solution was used for conditioning the aversion did not generalise to concentrations below 2 mg/ml. The characteristics of taste aversions conditioned with orally consumed amphetamine are similar to those of conditioning involving injections of the drug.     

Rewarding and aversive properties of IP and SC cocaine: assessment by place and taste conditioning

Three experiments were conducted to compare the effectiveness of intraperitoneally (IP) administered or subcutaneously (SC) administered cocaine to produce place and/or taste conditioning after four conditioning trials. In each experiment, IP (5–20 mg/kg) cocaine produced a place preference, but SC (0.5–20 mg/kg) cocaine at concentrations that prevented necrosis, did not produce a place preference. The failure of SC cocaine to produce a place preference was not a function of conditioning trial duration. On the other hand, SC cocaine (20 mg/kg) produced conditioned taste avoidance, but IP cocaine (20 mg/kg) did not produce conditioned taste avoidance. The results suggest that IP cocaine, but not SC cocaine, is rewarding.