Effects of E-2078, a stable dynorphin A(1–8) analog, on sedation and serum prolactin levels in rhesus monkeys

Rationale: In previous comparisons with C57BL/6J mice, DBA/2J mice have been characterized as ”hyporesponsive” to cocaine’s rewarding effect in the conditioned place-preference paradigm. This finding contrasts with other studies showing greater sensitivity of DBA/2J mice to the rewarding effects of ethanol and morphine in the place conditioning task. Objectives: The purpose of the present study was to examine cocaine- induced place conditioning in both strains using apparatus and procedures similar to those used previously to assess ethanol and morphine preference conditioning. Methods: Mice from both strains were exposed to an unbiased place-conditioning procedure using 1, 10, or 30 mg/kg cocaine. Conditioning trial duration was 15, 30, or 60 min. Results: In general, C57BL/6J mice displayed a significant conditioned place preference that was relatively unaffected by cocaine dose or trial duration. In contrast, DBA/2J mice showed no place conditioning at the shortest trial duration, but an increasing level of preference as trial duration increased. At the longest trial duration, both strains showed similar levels of place preference. Conclusions: Genetic differences in sensitivity to cocaine’s rewarding effect depend critically on temporal parameters of the place-conditioning procedure. One possible interpretation of these findings is that short trial durations produce conditioned activity responses that interfere more with expression of conditioned place preference in DBA/2J mice than in C57BL/6J mice. More generally, these findings underscore the need for caution when drawing conclusions about genetic differences in place conditioning, especially when using this paradigm to evaluate the effects of gene knockouts or insertions on drug reward. Received: 31 December 1998 / Final version: 15 April 1999     

Localization of genes influencing ethanol-induced conditioned place preference and locomotor activity in BXD recombinant inbred mice

Genetic differences in ethanol's ability to induce conditioned place preference were studied in 20 BXD Recombinant Inbred (RI) mouse strains and in the C57BL/6J and DBA/2J progenitor strains. Male mice from each strain were exposed to a Pavlovian conditioning procedure in which a distinctive floor stimulus (CS+) was paired four times with ethanol (2 g/kg). A different floor stimulus (CS-) was paired with saline. Control mice were injected only with saline. Floor preference testing without ethanol revealed significant genetic differences in conditioned place preference, with some strains spending nearly 80% time on the ethanolpaired floor while others spent only 50% (i.e., no preference). Control mice showed genetic differences in unconditioned preference for the floor cues, but unconditioned preference was not genetically correlated with conditioned preference. There were also substantial genetic differences in ethanol-stimulated activity, but contrary to psychomotor stimulant theory, ethanol-induced activity on conditioning trials was not positively correlated with strength of conditioned place preference. However, there was a significant negative genetic correlation (r=–0.42) between test session activity and preference. Quantitative trait loci (QTL) analyses showed strong associations (P<0.01) between conditioned place preference and marker loci on chromosomes 4, 8, 9, 18 and 19. Weaker associations (0.01<P<0.05) were identified on several other chromosomes. Analysis also yielded several significant QTL for unconditioned preference, ethanol-stimulated activity, and sensitization. Overall, these data support the conclusion that genotype influences ethanol-induced conditioned place preference, presumably via genetic differences in sensitivity to ethanol's rewarding effects. Moreover, several chromosomal regions containing candidate genes of potential relevance to ethanol-induced conditioned place preference have been identified.     

Forebrain PENK and PDYN gene expression levels in three inbred strains of mice and their relationship to genotype-dependent morphine reward sensitivity

Rationale

Vulnerability to drug abuse disorders is determined not only by environmental but also by genetic factors. A body of evidence suggests that endogenous opioid peptide systems may influence rewarding effects of addictive substances, and thus, their individual expression levels may contribute to drug abuse liability.

Objectives

The aim of our study was to assess whether basal genotype-dependent brain expression of opioid propeptides genes can influence sensitivity to morphine reward.

Methods

Experiments were performed on inbred mouse strains C57BL/6J, DBA/2J, and SWR/J, which differ markedly in responses to morphine administration: DBA/2J and SWR/J show low and C57BL/6J high sensitivity to opioid reward. Proenkephalin (PENK) and prodynorphin (PDYN) gene expression was measured by in situ hybridization in brain regions implicated in addiction. The influence of the κ opioid receptor antagonist nor-binaltorphimine (nor-BNI), which attenuates effects of endogenous PDYN-derived peptides, on rewarding actions of morphine was studied using the conditioned place preference (CPP) paradigm.

Results

DBA/2J and SWR/J mice showed higher levels of PDYN and lower levels of PENK messenger RNA in the nucleus accumbens than the C57BL/6J strain. Pretreatment with nor-BNI enhanced morphine-induced CPP in the opioid-insensitive DBA/2J and SWR/J strains.

Conclusions

Our results demonstrate that inter-strain differences in PENK and PDYN genes expression in the nucleus accumbens parallel sensitivity of the selected mouse strains to rewarding effects of morphine. They suggest that high expression of PDYN may protect against drug abuse by limiting drug-produced reward, which may be due to dynorphin-mediated modulation of dopamine release in the nucleus accumbens.     

Effect of Ro 15-4513 on ethanol-induced conditioned place preference.

The benzodiazepine receptor inverse agonist Ro 15-4513 reverses a number of ethanol's effects, including its reinforcing properties as measured through self-administration. The present study examined the effect of this putative ethanol antagonist in a place conditioning design that has been shown to be sensitive to ethanol's rewarding properties in mice. Using an unbiased differential conditioning procedure, DBA/2J mice received, on alternate days, pairings of a distinctive floor stimulus (CS+) with either ethanol (2 g/kg), Ro 15-4513 (3 mg/kg), or a combination of ethanol and Ro 15-4513. On alternate days, a different distinctive floor stimulus (CS-) was paired with vehicle. Under these conditions, ethanol produced a conditioned place preference that was unaffected by Ro 15-4513. Ro 15-4513 alone did not produce either a place preference or aversion. Ro 15-4513 did produce reductions in locomotor activity during conditioning, indicating it was behaviorally active. These results indicate that a dose of Ro 15-4513 that alters general activity does not affect ethanol reward.     

Susceptibility to amphetamine-induced place preference is predicted by locomotor response to novelty and amphetamine in the mouse

Rationale It has been demonstrated that major differences between mice of the C57BL/6J and DBA/2J inbred strains for amphetamine-induced place conditioning (preference and avoidance, respectively) are evident in standard housing conditions but abolished by temporary restricted feeding. This gene-experience model may be usefully exploited to dissect behavioral phenotypes related to place conditioning induced by addictive drugs.Objectives This study evaluated a number of behavioral phenotypes related to amphetamine-induced place preference for strain differences (C57BL/6J vs DBA/2J) susceptible to be abolished by temporary food restriction.Methods Mice of the two inbred strains were tested for: (1) conditioned taste aversion and place preference induced by amphetamine within the same dose-range; (2) preference for a novel compartment 24 h after a single exposure to only one of two compartments; (3) amphetamine-induced behavioral sensitization and conditioned hyperactivity; and (4) locomotor activity during exploration of a novel environment.Results The two strains showed consistent taste aversion at doses of amphetamine that promoted opposite strain-dependent place conditioning. Both strains spent more time exploring the novel rather than the known compartment of the place conditioning apparatus. Instead, only mice of the C57 strain showed amphetamine-induced behavioral sensitization and conditioned hyperactivity. However, temporary food restriction did not affect strain differences for these phenotypes. Finally, C57 mice were more active than DBA in a novel environment and restricted feeding abolished this strain-dependent difference.Conclusions These results relate individual differences for amphetamine-induced place conditioning with locomotor response to amphetamine and novelty.     

DBA/2J mice develop stronger lithium chloride-induced conditioned taste and place aversions than C57BL/6J mice

Genetic differences in lithium-induced conditioned aversion were examined using both place- and taste-conditioning procedures. In the place-conditioning procedure, adult male C57BL/6J (B6) and DBA/2J (D2) mice were exposed to a differential conditioning procedure in which each mouse received four 30-min pairings of a distinctive floor cue immediately after IP injections of either 0.75, 1.5, or 3. 0 mEq/kg LiCl. A different floor cue was paired with saline injections. A separate group of control mice received saline injections paired with both floor types. Subsequent floor preference testing revealed greater conditioned aversion in D2 mice compared to B6 mice in groups receiving 3.0 mEq/kg LiCl. Lower LiCl doses did not produce conditioning in either strain. In a conditioned taste-aversion procedure, fluid-restricted mice received four trials in which access to 0.2 M NaCl solution was followed by IP injection of either 0.75, 1.5, 3.0, or 6.0 mEq/kg LiCl. D2 mice showed stronger conditioned taste aversion than B6 mice at all doses, suggesting that taste conditioning may be a more sensitive index of aversive drug sensitivity than place conditioning. These findings are not well explained by strain differences in general learning ability or by strain differences in stimulus salience or innate preference. Rather, these data appear more consistent with previous studies showing strain differences in lithium pharmacokinetics and in general sensitivity to aversive events.     

Conditioned place preference after single doses or "binge" cocaine in C57BL/6J and 129/J mice

The rewarding effect of cocaine as reflected by the development of conditioned place preference was examined in C57BL/6J and 129/J mice. Cocaine was administered in a single daily dose (2.5, 5, 10 and 20 mg/kg ip) or in a "binge" pattern (15 mg/kg ip x3, hourly). Mice remained in the conditioning compartment for 30 min immediately after each injection. Single injections of cocaine from 5 to 20 mg/kg induced conditioned place preference in each strain of mice. Only C57BL/6J mice developed conditioned place preference after "binge" cocaine administration. Both strains showed significantly greater locomotion in the conditioning compartment across the range of single doses of cocaine and after "binge" cocaine administration, but only 129/J mice showed sensitization. When mice that had received the single 10 mg/kg dose were retested 4 weeks later, the amount of time spent in the preferred side was significantly reduced compared to the initial test in the 129/J, but not in C57BL/6J mice. Thus, the persistence of conditioned place preference is strain dependent. The fact that 129/J mice did not develop conditioned place preference after "binge" cocaine administration, but did after single doses, suggests that the rewarding effects of cocaine are influenced by pattern of administration, a factor that may be relevant to the development of human cocaine addiction.     

Role of test activity in ethanol-induced disruption of place preference expression in mice

Rationale Reduced expression of a drug-induced conditioned place preference (CPP) may reflect a decrease in the drug’s conditioned rewarding effects. However, CPP is also open to disruption by processes unrelated to the underlying motivation. In unpublished studies, we previously observed that ethanol pretreatment before testing disrupted expression of ethanol-induced CPP in DBA/2J mice. We hypothesized that this interference effect was due to large ethanol-induced increases in activity. Objective The present studies were designed to examine the relationship between test activity and expression of ethanol-induced CPP both in the presence and absence of ethanol. To assess the generality of this relationship, we examined these effects both in DBA/2J (which are highly activated by ethanol) and in NZB/B1NJ mice (which show similar CPP, but less ethanol-induced activation). Materials and methods In separate experiments, inbred mice from each strain underwent ethanol (2 g/kg) place conditioning. Saline or ethanol was then administered immediately before the test. Results Ethanol, given immediately before the test, blocked the expression of ethanol CPP in DBA/2J, but not in NZB/B1NJ mice. Moreover, ethanol significantly increased test activity levels in DBA/2J and to a much lesser degree in NZB/B1NJ mice. Correlation analyses showed an inverse phenotypic relationship between preference and test activity, reflecting stronger preferences in less active mice. Conclusions Disruption of ethanol-CPP observed in DBA/2J mice may be a consequence of high ethanol-induced activity levels. More generally, these studies suggest that competing behaviors can affect expression of a drug-induced CPP independent of affecting the conditioned rewarding effects of the drug.     

Acquisition and expression of ethanol-induced conditioned place preference in mice is inhibited by naloxone

The effects of opioid antagonists on conditioned reward produced by ethanol provide variable and sometimes conflicting results, especially in mice. In the present set of experiments, male C57BL/6 mice received 4 vehicle and 4 ethanol conditionings, and the rewarding effects of ethanol were assessed in an unbiased version of the conditioned place preference (CPP) apparatus and an unbiased stimulus assignment procedure. Intraperitoneal (ip) administration of ethanol (2 g/kg, but not 1 g/kg) resulted in the conditioned reward when conditionings lasted for 6 min but not when conditioning lasted for 20 min. Administration of the non-selective opioid receptor antagonist naloxone (1 and 5 mg/kg) before the conditionings attenuated the acquisition of ethanol-induced place preference. Naloxone (1 mg/kg) also inhibited expression of the CPP response, but it did not alter the preference of vehicle-conditioned mice, suggesting the lack of its own motivational effects in this experimental setting. Taken together, the present results suggest that an unbiased version of ethanol-induced CPP in C57BL/6 mice could be a valid model for the study of the motivational effects of ethanol, confirming and expanding previous findings that have demonstrated inhibitory effects of opioid receptor antagonist on alcohol conditioned reward.     

Evidence for the rewarding effects of ethanol using the conditioned place preference method

Rats were tested for the rewarding effects of ethanol using a place preference conditioning procedure. After receiving a total of 15 daily conditioning trials under 1.0 g/kg ethanol (IP), a significant place preference was produced. Subjects conditioned using saline or 0.5 g/kg ethanol showed no changes in place preference. This study suggests that failures to demonstrate rewarding effects from ethanol with the conditioned place preference method may be due to an insufficient number of conditioning trials or to an inadequate exposure to the drug. The fact that place preference conditioning was effective in demonstrating ethanol reward while other methods have been equivocal suggests that this method may be a valuable technique for studying the mechanisms of ethanol reward.     

Haloperidol reduces ethanol-induced motor activity stimulation but not conditioned place preference

This experiment examined the impact of a dopamine receptor blocker on ethanol's rewarding effect in a place conditioning paradigm. DBA/2J mice received four pairings of a tactile stimulus with ethanol (2 g/kg, IP), haloperidol (0.1 mg/kg, IP) + ethanol, or haloperidol alone. A different stimulus was paired with saline. Ethanol produced increases in locomotor activity that were reduced by haloperidol. However, conditioned preference for the ethanol-paired stimulus was not affected by haloperidol. Haloperidol alone decreased locomotor activity during conditioning and produced a place aversion. These results indicate a dissociation of ethanol's activating and rewarding effects. Moreover, they suggest that ethanol's ability to induce conditioned place preference is mediated by nondopaminergic mechanisms.     

Enhanced ethanol-, but not cocaine-induced, conditioned place preference in Apoe(-/-) mice

Apolipoprotein (apo) E is a glycoprotein that is most commonly associated with cardiovascular and Alzheimer's disease risk. Recent data showing that apoE mRNA expression is reduced in the frontal cortex of alcoholics raise the possibility that apoE may also be related to the rewarding properties of ethanol. In this study, we examined whether Apoe deletion affects the rewarding properties of ethanol in mice. Male and female wild-type (WT; C57BL/6J) and apoE knockout (Apoe(-/-); C57BL/6J-Apoe(tm1Unc)) mice underwent an unbiased place conditioning procedure with ethanol (2 g/kg) or cocaine (5 mg/kg). Female mice were also tested for ethanol intake in a two-bottle choice procedure. Apoe(-/-) mice showed greater ethanol-induced conditioned place preference (CPP). In contrast, cocaine-induced CPP and ethanol intake were similar between the genotypes. These findings suggest that apoE normally reduces the conditioned rewarding properties of ethanol but not of cocaine. While the exact mechanisms underlying these effects of apoE are unknown, these data support a possible role for apoE in modulating the conditioned rewarding properties of ethanol.     

The anti-relapse compound acamprosate inhibits the development of a conditioned place preference to ethanol and cocaine but not morphine

The effects of the anti-relapse compound acamprosate (calcium acetylhomotaurinate) on the conditioned rewarding effects of ethanol, cocaine and morphine were studied using the conditioned place preference (CPP) paradigm. During 3 days of drug conditioning, mice were pretreated with saline or acamprosate (30, 100 or 300 mg kg(-1) i.p.) 10 min prior to the administration of ethanol (2 g kg(-1) i.p.), cocaine (15 mg kg(-1) i.p.) or morphine (10 mg kg(-1) i.p.), and subsequently confined to one of two distinct conditioning chambers. On the following day, mice were tested for the expression of CPP. Acamprosate dose-dependently reduced the development of CPP to ethanol and cocaine but not morphine. When tested as the conditioning drug, acamprosate alone produced neither a conditioned place preference nor aversion. These data suggest that acamprosate can suppress the conditioned rewarding effects of ethanol and certain classes of abused substances.     

Susceptibility to conditioned place preference induced by addictive drugs in mice of the C57BL/6 and DBA/2 inbred strains

Rationale In previous studies, we have demonstrated that mice of the inbred strain C57BL/6J (C57) are more susceptible to amphetamine-induced conditioned place preference (CPP) than DBA/2J (DBA) mice. Moreover, we also observed parallel strain differences for the locomotor-stimulant effects of the drug. However, other studies have reported either no difference or opposite strain differences for cocaine- and morphine-induced CPP as well as for the locomotor effects of these drugs, suggesting that amphetamine-related behavioral phenotypes might depend on a specific pharmacological action of the psychostimulant. Objectives This study was aimed at testing strain differences for cocaine- and morphine-related behavioral phenotypes in the same experimental protocol and conditions previously used for amphetamine. Methods C57 and DBA mice were tested for CPP induced by cocaine (0, 5, 10, and 20 mg/kg) and morphine (0, 5, 7.5, and 10 mg/kg). Locomotor activity data were simultaneously obtained by measuring distance moved during all different CPP phases and unconditioned locomotor activity, behavioral sensitization and conditioned hyperactivity were measured together with CPP. Results (a) Either cocaine or morphine promoted significant CPP at lower doses in C57 than in DBA mice; (b) only drug-trained C57 mice showed a significant CPP compared with the control group; and (c) only C57 mice showed dose-dependent effects of cocaine on CPP. Moreover, there was no relationship between drug-induced CPP and locomotion. Conclusions The results demonstrate that C57 and DBA mice differ in their sensitivity to cocaine- and morphine-induced CPP and suggest that the two strains differ in sensitivity to the positive incentive properties of drugs of abuse.     

Dopamine D3 receptor knockout mice and the motivational effects of ethanol

Dopamine D3 receptors have been implicated in the behavioral effects of abused drugs including ethanol. The present experiments characterized the acquisition of ethanol-induced place conditioning and ethanol self-administration in D3 knockout (D3 KO) mice compared with C57BL/6J (C57) mice. For place conditioning, D3 KO and C57 mice received six pairings of a tactile stimulus with ethanol (3 g/kg i.p.). D3 KO mice showed higher basal locomotor activity levels in comparison with the C57 mice during conditioning. Ethanol produced similar magnitudes of conditioned place preference in both genotypes. In a two-bottle drinking procedure, mice of each genotype received 24 h access to water and either 3% or 10% v/v ethanol. No difference was noted between D3 KO and C57 mice in either consumption or preference. In an operant self-administration procedure using 23 h sessions, D3 KO and C57 mice received access to 10% v/v ethanol on an FR4 schedule of reinforcement, food on an FR1 schedule of reinforcement and water from a sipper tube. D3 KO and C57 mice had similar response rates of ethanol and food as well as similar water intakes. Overall, these results indicate that elimination of D3 receptor function has little influence on ethanol reward or intake.     

Qualitative differences between C57BL/6J and DBA/2J mice in morphine potentiation of brain stimulation reward and intravenous self-administration

Rationale

The C57BL/6J (C57) and DBA/2J (DBA) mice are the most common genotypes used to identify chromosomal regions and neurochemical mechanisms of interest in opioid addiction. Unfortunately, outside of the oral two-bottle choice procedure, limited and sometimes controversial evidence is available for determining their relative sensitivity to the rewarding effects of morphine.

Objectives

The purpose of this study was to utilize classically accepted models of drug abuse liability to determine relative susceptibility to the rewarding effects of morphine.

Methods

The ability of morphine or amphetamine to potentiate lateral hypothalamic brain stimulation and intravenous morphine self-administration (across three doses in a fixed ratio schedule and at the highest dose in progressive ratio schedules) was investigated in both genotypes.

Results

In both measures, C57 and DBA mice differed dramatically in their response to morphine. Morphine potentiated rewarding stimulation in the C57 mice but antagonized it in the DBA mice. Consistent with these findings, intravenous morphine did not serve as a positive reinforcer in DBA mice under conditions that were effective in the C57 mice using a fixed ratio schedule and failed to sustain levels of responding sufficient to maintain a constant rate of drug intake under a progressive ratio schedule. In contrast, amphetamine potentiated the rewarding effects of brain stimulation similarly in the two genotypes.

Conclusions

These findings provide strong evidence that morphine is rewarding in the C57 genotype and not in the DBA genotype. Understanding their relative susceptibility is important given the prominence of these genotypes in candidate gene identification and gene mapping.     

Conditioned activation induced by ethanol: role in sensitization and conditioned place preference.

Previous studies of ethanol-induced activation and place preference conditioning have shown that repeated exposure to ethanol produces sensitization to ethanol's locomotor activating effect in mice. This experiment was designed to determine whether the behavioral sensitization to ethanol that occurs during place preference conditioning is due to development of a Pavlovian conditioned activity response. Mice (DBA/2J) in the experimental group (BEFORE) received four pairings of a distinctive floor stimulus with ethanol (2 g/kg, IP); a different floor stimulus was paired with saline (counterbalanced). Mice in two control groups were exposed equally to each floor stimulus and were handled and injected as often as experimental mice. One control group (AFTER) always received ethanol in the home cage 1 h after exposure to the floor stimulus, while the other control group (NO-DRUG) never received ethanol during conditioning. BEFORE group mice showed a significant conditioned place preference, whereas control mice did not. Activity tests after saline or ethanol indicated higher activity levels in BEFORE mice compared to control mice, regardless of floor stimulus. Moreover, BEFORE mice were more active on their CS+ floor than on their CS- floor during saline tests; activity was equally elevated on both floors during ethanol tests. These results support the hypothesis that sensitization to ethanol's activating effect is mediated by Pavlovian conditioning. Further, they suggest that place conditioning established-associative control by two kinds of stimuli; the specific tactile cues serving as CS+ and CS- and the general environmental cues common to both CS+ and CS- trials.(ABSTRACT TRUNCATED AT 250 WORDS)     

Baclofen alters ethanol-stimulated activity but not conditioned place preference or taste aversion in mice.

The present experiments examined the effects of the GABA(B) receptor agonist, baclofen, on the acquisition of ethanol-induced conditioned place preference (CPP) and conditioned taste aversion (CTA) in male DBA/2J mice. Mice in the CPP experiment received four pairings of ethanol (2g/kg) with a distinctive floor stimulus for a 5-min conditioning session (CS+ sessions). On intervening days (CS- sessions), mice received saline injections paired with a different floor type. On CS+ days, mice also received one of four doses of baclofen (0.0. 2.5, 5.0, or 7.5 mg/kg) 15 min before an injection of ethanol. For the preference test, all mice received saline injections, and were placed on a half-grid and half-hole floor for a 60-min session. Baclofen dose dependently reduced ethanol-stimulated activity, but did not alter the magnitude of ethanol-induced CPP at any dose. For the CTA experiment, mice were adapted to a 2-h per day water restriction regimen followed by five conditioning trials every 48 h. During conditioning trials, subjects received an injection of saline or baclofen (2.0 and 6.0 mg/kg) 15 min before injection of 2 g/kg ethanol or saline following 1-h access to a saccharin solution. Baclofen did not alter the magnitude of ethanol-induced CTA at any dose. In addition, baclofen alone did not produce a CTA. Overall, these studies show that activation of GABA(B) receptors with baclofen reduces ethanol-induced locomotor activation, but does not alter ethanol's rewarding or aversive effects in the CPP and CTA paradigms in DBA/2J mice.     

Ethanol-induced conditioned place preference, but not aversion, is blocked by treatment with d-penicillamine, an inactivation agent for acetaldehyde

Rationale There is evidence to suggest that acetaldehyde is involved in the control of ethanol-seeking behavior and reward. d-penicillamine, a thiol amino acid, is a highly selective agent for the inactivation of acetaldehyde. Previous studies from our laboratory have demonstrated that d-penicillamine prevents both behavioral stimulation induced by ethanol and acetaldehyde-produced locomotor depression in mice. Objectives The contribution of ethanol-derived acetaldehyde to the affective effects of ethanol (preference and aversion) was assessed using an unbiased place conditioning design. Methods Male mice received four pairings of a distinctive floor stimulus (CS+: GRID+ or HOLE+) with injections of saline and ethanol (2 g/kg) given before (preference) or after (aversion) the 5-min exposure to the place conditioning apparatus. A different floor stimulus (CS−: GRID− or HOLE−), associated with saline-saline injections on alternate days, was presented. For a different group of animals, the pairings with the CS+ were associated with saline and ethanol injections, but on alternate days, they received d-penicillamine (50 or 75 mg/kg) and ethanol injections paired with the CS−floor stimulus. A 60-min preference test was carried out 24 h after the last conditioning trial. A similar procedure was followed to test the effect of d-penicillamine on morphine (16 mg/kg) and cocaine-induced (20 mg/kg) conditioned place preference (CPP). Results CPP and conditioned place aversion (CPA) were observed for ethanol, but d-penicillamine only blocked CPP. d-penicillamine, by itself, did not produce either rewarding or aversive effects. CPP observed for morphine and cocaine was unaffected by d-penicillamine pretreatment. Conclusions The results of the present study suggest that the selective inactivation of acetaldehyde blocked the rewarding, but not aversive, effects of ethanol and support the role of this ethanol metabolite in the affective properties of ethanol.