Naloxone-precipitated conditioned taste aversions in morphine-dependent Fischer (F344) and Lewis rat strains

The Fischer 344 (F344) and Lewis (LEW) rat strains are genetically divergent populations that are used to study the effects of and responses to drugs of abuse. In this context, LEW rats display faster acquisition of drug self-administration than F344 rats. Interestingly, these strains have also been reported to differ in their somatic responses to morphine withdrawal. To address possible strain differences in the affective response to withdrawal, the present study assessed the ability of naloxone-precipitated withdrawal from morphine to induce conditioned taste aversions in male F344 and LEW rats. Specifically, subjects from each of these strains were given chronic morphine to induce dependence and then given access to a novel saccharin solution followed by naloxone. These pairings were given every fourth day for a total of two conditioning trials after which subjects were given access to saccharin but without naloxone administration to assess extinction of the naloxone-induced aversion. Behavioral assays of withdrawal were also performed after each naloxone administration. Both F344 and LEW subjects acquired aversions to the naloxone-associated taste with no significant differences in the rate of acquisition of the aversions. Differences did appear during extinction with LEW animals extinguishing the taste aversion significantly faster than F344 animals. The data were discussed in terms of the relative strength of the affective responses during withdrawal and the role of such responses to drug use and abuse.     

Temporal analysis of naloxone attenuation of morphine-induced taste aversion.

In a dose-response study, 7.5 mg/kg of naloxone produced maximal attenuation of conditioned taste aversion to saccharin induced by 10 mg/kg of morphine. Naloxone was administered immediately after the morphine in this study. In a second experiment, naloxone still caused a significant attenuation of taste aversions when administered with a 1 hr delay after morphine, but not after delays of 4 or 8 hr. These results suggest that behavioral consequences of morphine which peak during the first hr after injection (analgesia, catalepsy, and depression of intracranial self-stimulation) are not correlated with the aversive effect of morphine. Nor can the aversiveness of morphine be attributed to withdrawal effects. Only the facilitative actions of morphine occurring 1 to 4 hr after injection, including the facilitation of intracranial self-stimulation, are temporally correlated with the naloxone-sensitive aversive effect. Thus, a temporal analysis cannot be used to dissociate the paradoxical positive reinforcement and aversive effects of morphine. Rather, the temporal correlation between the two opposite motivational effects of morphine serves to emphasize the nature of this paradox.     

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.     

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.     

Conditioned flavor aversions for assessing precipitated morphine abstinence in rats.

Rats receiving twice-daily morphine injections acquired aversions to a saccharin solution which had been presented for 1 hr immediately prior to injections of naloxone. The degree of aversion was related to the maintenance dosage of morphine. Rats maintained on a regimen of daily saline injections did not show significant aversion to saccharin paired with naloxone, even at doses as high as 40 mg/kg. The sensitivity of the technique was such that significant aversions could be demonstrated in rats receiving doses of morphine as low as 1 mg/kg twice daily. It is suggested that conditioned flavor aversions provide a useful method for assessing the aversive quality of abstinence precipitated from low doses of morphine.     

Attenuation of cyclophosphamide-induced taste aversions in mice by prochlorperazine, delta 9-tetrahydrocannabinol, nabilone and levonantradol

A series of experiments were performed with adult CD-1 male mice to evaluate the antiemetic effects of several compounds using the conditioned taste aversion procedure. The antiemetics were administered IP immediately prior to a 30-min conditioning trial in which a novel tasting solution (0.3% saccharin) was presented to the subjects. The emetics, apomorphine and the cancer chemotherapeutic drug cyclophosphamide, were given IP immediately after the conditioning trial at doses that induced taste aversions. Three days later the mice received a two bottle preference test (saccharin vs. water) and the percent saccharin consumed of the total fluid intake was calculated. Doses of the phenothiazine antiemetic prochlorperazine (1 and 3 mg/kg) attenuated the aversions produced by 0.3 and 1.0 mg/kg apomorphine. Doses of drugs currently approved or under clinical investigation as antiemetics in conjunction with cancer chemotherapy, i.e., prochlorperazine (1.0 mg/kg), delta 9-tetrahydrocannabinol (0.3 and 1.0 mg/kg) and nabilone (0.01 and 0.03 mg/kg), significantly attenuated the taste aversions induced by cyclophosphamide. Levonantradol at doses of 0.03 and 0.06 mg/kg, however, did not attenuate cyclophosphamide-induced taste aversions. Conditioned taste aversions produced by emetic drugs warrants investigation as a model for evaluating potential antiemetics.     

Heroin and morphine: Aversive and analgesic effects in rats

Although a number of studies demonstrate morphine-induced taste aversions, no such reports exist for heroin. In a conventional taste aversion paradigm, rats were injected with one of six heroin doses (0.5–12.0 mg/kg) after consuming a novel saccharin solution (Experiment 1). When the saccharin was reintroduced a second time no significant reduction in consumption occuredd at any of the doses tested. It was therefore concluded that heroin does not readily induce a taste aversion. In experiment 2, morphine was tested in an identical taste aversion paradigm and, as expected, a significant taste aversion.did result at two of the doses tested. Experiment 3 demonstrated that heroin produced analgesia equal to or greater than morphine when comparing dosages of heroin which failed to induce a CTA with CTA-inducing morphine dosages. Thus, whereas heroin is more potent than morphine as an analgesic, heroin is less potent than morphine as a CTA-inducing agent.     

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.     

Aversive conditioning properties of caffeine in rats

Four experiments tested the conditioning effects of caffeine. Flavor and place cues were paired with IP caffeine injections and followed by tests for cue preference. In Experiment 1A, saccharin was paired with 1.25, 5 or 20 mg/kg of caffeine. In Experiment 1B, caffeine was delivered 30 min before, 5 min before, or 30 min after saccharin. Dose- and time-dependent conditioned taste aversions were produced. In Experiment 2, a place and taste cue were paired simultaneously with 5 or 20 mg/kg of caffeine. Conditioned place and taste aversions developed at 20, but not at 5 mg/kg. In Experiment 3, a place cue alone was paired with 0, 5, or 20 mg/kg of caffeine; dose-dependent conditioned place aversions developed. In Experiment 4, place and taste cues were paired with control treatments: pH-buffered caffeine, purine or vehicle. Caffeine produced taste aversions whereas the purine and vehicle did not. These aversive conditioning effects of caffeine across a variety of situations, doses and temporal arrangements stand in contrast to results obtained with other psychoactive drugs, such as amphetamine and alcohol.     

Withdrawal from chronic nicotine fails to produce a conditioned taste aversion to saccharin in rats

Sprague-Dawley rats were maintained on a daily regimen of nicotine, morphine or saline administration for 28 days. Following the discontinuation of the daily drug regimen, rats were given a choice of tap water or a saccharin-water solution. The rats previously receiving morphine drank significantly less saccharin-water solution than did the rats receiving nicotine or saline injections. The failure of the nicotine rats to display a conditioned aversion to the novel saccharin flavor suggests that nicotine did not produce a physiological withdrawal syndrome analogous to morphine withdrawal in this paradigm.     

Effects of parachlorophenylalanine on ethanol self-selection in the rat.

The efficacy of p-chlorophenylalanine (PCPA) in producing conditioned taste aversions and unconditioned avoidance of ethanol was investigated in two experiments. It was found that administering PCPA to rats having free access to a saccharine solution and water produced robust aversions to saccharin that extinguished within 6 days after termination of the PCPA treatments, thereby indicating that PCPA can produce conditioned aversions to substances consumed during its administration. In a second experiment, intraperitoneal injections of PCPA and/or ethanol given to rats not having access to ethanol were found to produce no change in their subsequent ethanol preferences. The results support the contention of earlier investigators that the reported of PCPA on the rat's preference for ethanol may have been due largely to the animals acquiring conditioned aversions to ethanol during PCPA treatments.     

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.     

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.     

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.     

Assessment of the aversive effects of peripheral mu opioid receptor agonism in Fischer 344 and Lewis rats

The Fischer 344 (F344) and Lewis (LEW) inbred rat strains differ on a host of biochemical, neuroanatomical, immunological and behavioral endpoints. One behavioral difference of interest is their differential reactivity to the aversive effects of morphine as indexed by the conditioned taste aversion preparation (aversions acquired by F344 rats are significantly greater than those acquired by the LEW strain). This differential effect appears to be specific to opioids that work primarily on the mu opioid receptor. Given that morphine works systemically, it is unknown whether these differential effects in F344 and LEW animals are centrally or peripherally mediated. To address this issue, the present study investigated the ability of the peripherally acting mu preferring opioid agonist loperamide to induce differential taste aversions in F344 and LEW animals. Both F344 and LEW animals acquired dose-dependent taste aversions to the loperamide-associated solution with no difference between them. Additionally, control animals initially injected with vehicle during aversion training with loperamide and subsequently conditioned with morphine displayed the typical aversive profile to morphine (F344 > LEW). Although the basis for the present data is unknown, their relation to morphine-induced taste aversions and the role of the interaction of stimulus effects of drugs that produce differential abuse liability were discussed.     

Second-order (environmental) conditioning of a taste aversion in rats

Four groups of rats (n = 10/group) were conditioned in a taste aversion task using a second-order reinforcer associated with precipitated morphine withdrawal. Rats in CS+, CS- and CS(random) groups were exposed to a chronic morphine (morphine sulfate, MS) dosing regimen. A control group received equivalent volumes of saline. All rats then received daily i.p. injections of naloxone HCI (1.0-3.2mg/kg), inducing precipitated morphine withdrawal in group-dependent unique environments. The 4-h withdrawal trials were terminated by a 20mg/kg MS injection (MS treatment groups only) and returned to their home cage. After a 1-week wash-out was imposed, all subjects were exposed to a conditioned taste aversion (CTA) task using environmental stimuli (CSI) from Phase 1 paired with saccharin (CS2) in a second-order conditioning procedure. The CS+ group developed a significant CTA; the CS- and CS(random) groups increased their consumption of saccharin. The saline group was unaffected by the treatment conditions. The data demonstrate the salience and importance of environmental stimuli and suggest a role for such conditioning in drug relapse phenomena.     

Conditioned taste aversions induced by phencyclidine and other antagonists of N-methyl-D-aspartate

Taste aversions can be conditioned in rats by a variety of psychoactive drugs, including those with reinforcing properties. Previous research, however, has not established clearly whether phencyclidine and related drugs are active in such procedures. The present study was carried out to investigate whether phencyclidine would induce a conditioned taste aversion and whether several other compounds (MK-801, the stereoisomers of NANM and ifenprodil) which, like phencyclidine, are known to antagonise the actions of N-methyl-D-aspartate (NMDA), would produce similar effects. When rats received injections of these compounds, after consuming a novel solution of saccharin, their subsequent consumption of the same solution decreased. The smallest doses of the different drugs which induced clear taste aversions were: phencyclidine 3 mg/kg, MK-801 0.3 mg/kg, (+)-NANM 10 mg/kg, (-)-NANM 3 mg/kg and ifenprodil 10 mg/kg. Thus, all the drugs were active. However, as neither the potencies nor the efficacies of the different compounds in inducing taste aversions correlated with their other behavioural effects or with their relative potencies in antagonising the effects of NMDA or in displacing phencyclidine from its binding sites, the mechanisms involved are unclear.     

Conditioned Sucrose Aversions Produced by Naloxone-Precipitated Withdrawal From Acutely Administered Morphine

The aversive properties of acute naloxone-precipitated morphine withdrawal were examined in the taste reactivity paradigm. Acute naloxone-precipitated withdrawal paired with sucrose solution established conditioned active rejection of the sucrose solution. Active rejection of sucrose was observed when naloxone was administered both 1 h and 22 h after morphine. When the stimulus properties of morphine were present during the conditioning trial, the conditioned sucrose aversion was only expressed when the rats were tested in the same drug state in which they had learned the aversion. However, when the stimulus properties of morphine were not present during conditioning, the aversion was expressed in the absence of the morphine state. The results suggest that palatability shifts can be conditioned to sucrose paired with acute morphine withdrawal.     

Assessment of SNC 80 and naltrindole within a conditioned taste aversion design

Although compounds with relative selectivity for the mu and kappa opiate receptors subtypes have been reported to condition taste aversions, it is not known whether systemically administered delta compounds have the ability to produce aversions. To that end, female Long-Evans rats were adapted to water deprivation and were given pairings of a novel saccharin solution and various doses of the selective delta agonist SNC 80 (0.32-10.0 mg/kg; Experiment 1) or the selective delta antagonist naltrindole (1.0-18.0 mg/kg; Experiment 2). For comparison, the relatively selective mu agonist morphine (Experiment 1) and mu antagonist naloxone (Experiment 2) were assessed under identical conditions. Both SNC 80 (Experiment 1) and naltrindole (Experiment 2) were effective as unconditioned stimuli within this design, inducing dose-dependent taste aversions with repeated conditioning trials. Although at no dose did animals injected with SNC 80 differ from those injected with morphine, aversions induced by SNC 80 were acquired at a faster rate than those induced by morphine. Subjects injected with naloxone drank significantly less than those injected with naltrindole at the 10 mg/kg dose, and aversions induced by naloxone at 5.6 and 10 mg/kg were acquired at a faster rate than those induced by naltrindole. Although the basis for opioid agonist- and antagonist-induced taste aversions is not known, the differences between aversions induced by SNC 80 and naltrindole and those induced by morphine and naloxone, respectively, may be a function of their relative selectivity for specific opiate receptor subtypes.     

Morphine preexposure facilitates morphine place preference and attenuates morphine taste aversion

Repeated morphine preexposure has been reported to enhance measures of morphine reward (conditioned place preference; CPP) and attenuate measures of morphine aversion (conditioned taste aversion; CTA). These effects are generally independently assessed, limiting the ability to determine if the enhancing and attenuating effects of morphine exposure are mediated by a common factor. To assess any potential relationship between these two effects, the present study examined the impact of morphine preexposure on these motivational properties of morphine using a combined CTA/CPP procedure in which the same animals receive concurrent taste and place conditioning. Specifically, male Sprague-Dawley rats were preexposed to morphine [5 mg/kg; subcutaneously (sc)] or equivolume drug vehicle. Following preexposure, animals were given saccharin to drink and injected with morphine sulfate (1 or 5 mg/kg sc) or drug vehicle (CTA). Immediately thereafter, they were placed on one side of a two-compartment chamber (CPP). On the next day, they were given water followed by injections of the drug's vehicle and then placed in the other compartment. There were four such conditioning cycles after each of which a CTA and CPP test were given. While preexposure to morphine attenuated morphine-induced CTAs, morphine-induced CPPs were enhanced within the same animals. These effects of morphine preexposure were dose- and time-dependent and parallel. These data indicate that the attenuating and sensitizing effects of morphine preexposure on taste aversions and place preferences, respectively, could be mediated by a common mechanism, although other possibilities for these effects of morphine preexposure remain.