Opioid antagonists block the acquisition of ethanol-mediated conditioned tactile preference in infant rats

It has been difficult to find conditioned preference for tactile cues paired with ethanol intoxication in rats. Toward understanding the ontogeny of ethanol reinforcement, we aimed at establishing a simple and reliable procedure for (1) assessing primary appetitive conditioning to ethanol in infant rats and (2) discerning the role the opioid system plays in ethanol-mediated conditioning at this age. Experiment 1 determined the parameters (i.e., dose, interval of conditioning) for assessing ethanol-mediated conditioning. Pups were then trained with differential Pavlovian conditioning (Experiments 2 and 3) in which ethanol intoxication (1.0–2.0 g/kg, intragastrically or intraperitoneally delivered) was paired with a tactile stimulus (sandpaper) while an alternative texture signaled the absence of ethanol's effects. Unpaired control conditions were also used. Tactile preferences were assessed after two conditioning sessions. Paired rats spent significantly more time on sandpaper than unpaired controls, an effect that was greater after intragastric administration of 1.0 than 2.0 g/kg ethanol. This effect was replicated in Experiments 4a and 4c and found to be inhibited by pretreatment with general (naloxone [NAL]) or specific (d-Pen-Cys-Tyr-d-Trp-Orn-Thr-Pen-Thr-NH2 [CTOP] and naltrindole) opioid antagonists. Blood ethanol levels at conditioning were not altered by NAL (Experiment 4b). The study outlines a procedure that reveals appetitive conditioning to ethanol by infant rats. The results are discussed in terms of a potential ethanol-induced activation of the endogenous opioid system during the onset of the intoxication process.     

Exposure to low and moderate doses of alcohol on late gestation modifies infantile response to and preference for alcohol in rats

Several studies in rats have found that maternal administration of low or moderate doses of ethanol result in fetal perception of the chemosensory and toxic effects of ethanol. This prenatal experience with the drug enhances the palatability of ethanol's flavor and increases ethanol consumption during infancy and adolescence. The acquired preference for ethanol seems to be a conditioned response established prenatally, by the association of ethanol's sensory and reinforcing aspects, the latter mediated by the opioid system. These results are in accordance with data of studies in humans, and should be taken into account for clinical studies analyzing the relationship between prenatal ethanol exposure and later ethanol abuse problems.     

Differential motivational properties of ethanol during early ontogeny as a function of dose and postadministration time.

While appetitive reinforcement effects of ethanol are easily detected in rat neonates, such phenomena rarely have been observed in older infants. Recently, Molina et al. [Molina, J. C., Ponce L. F., Truxell, E., & Spear N. E. (2006). Infantile sensitivity to ethanol's motivational effects: ethanol reinforcement during the third postnatal week. Alcohol Clin Exp Res 30, 1506-1519] reported such effects of ethanol in 14-day-olds using a second-order conditioning procedure. Infants also appear to be sensitive to biphasic reinforcement or general motivational effects of ethanol, with appetitive effects seeming to occur early in the state of intoxication and aversive effects predominant during late stages, but tests have been inconclusive. The present study examined the possibility of biphasic motivational effects of ethanol during infancy through the use of second-order conditioning procedures. Preweanling rats (14 days old) experienced intraoral water infusions (conditioned stimulus, CS) either 5-20 or 30-45 min after administration of 0.5 or 2.0 g/kg i.g. ethanol. Pups were then exposed to the CS while over a novel texture (second-order phase). Tests of tactile preference for that texture followed. Locomotive, thermal, hormonal (corticosterone release), and pharmacokinetic patterns likely to underlie the acquisition of ethanol-mediated conditioning were also examined in subsequent experiments. Intraoral CSs paired with either early or late effects of low-dose ethanol (0.5 g/kg, blood ethanol concentration: 40 mg%) became positive second-order reinforcers. Appetitive effects were also exhibited by pups exposed to the CS during commencement of the toxic episode induced by a 2.0 g/kg ethanol dose, 5-20 min after administration of ethanol, whereas aversions emerged when CS presentation occurred 30-45 min postadministration time (blood ethanol concentrations: 157 and 200 mg%, respectively). Overall, the results indicate that infants rapidly detect differential motivational properties of ethanol as a function of dose or drug postadministration time. Relatively neutral stimuli associated with these properties are later capable of acting as either positive or aversive reinforcers. Thermal and motor responses that accompany ethanol intoxication do not seem to be directly associated with differential hedonic properties of the drug at this stage of development.     

Classically conditioned ethanol stimulus control of a motor behavior

Rats with extensive unilateral 6-hydroxydopamine lesions of substantia nigra rotate (turn in circles) in response to administration of dopamine agonists. Here apomorphine administration, but not single or repeated administrations of ethanol, resulted in rotation in such lesioned animals. When apomorphine and ethanol were administered simultaneously to lesioned animals, rapid contralateral rotation resulted. After five of these conditioning trials in which ethanol and apomorphine were paired, ethanol alone elicited apomorphine-like rotation. Saline administration did not result in rotation in the conditioned animals, indicating that ethanol rather than the injection procedure exerted stimulus control over the behavior. Control animals which were treated five times with apomorphine and ethanol in an unpaired manner showed no conditioned rotation to ethanol.     

Conditioned place preference induced by intraventricular infusions of acetaldehyde

Multiple intracerebroventricular infusions of acetaldehyde were shown to induce conditioned place preference in laboratory rats. Those animals receiving acetaldehyde infusions displayed an increased preference for environmental cues previously paired with the drug administration. These results indicate that acetaldehyde, when present in the brain, may have reinforcing properties.     

Acetaldehyde-reinforcing effects: differences in low-alcohol-drinking (UChA) and high-alcohol-drinking (UChB) rats.

It has been suggested that acetaldehyde has a biphasic effect on voluntary alcohol consumption. At low brain concentration, it might exert reinforcing effects, whereas high acetaldehyde levels would be predominantly aversive. The objective of the current study was to compare the effect of an intraperitoneal dose of acetaldehyde (50 mg/kg) in high-alcohol-drinking (UChB) and low-alcohol-drinking (UChA) rat lines, which differ in the activity of the brain mitochondrial class 2 aldehyde dehydrogenase (ALDH2) as a consequence of differences in their ALDH2 genotypes. A classical place-conditioning procedure was used to determine the reinforcing or aversive (or both) effects of acetaldehyde in ethanol-naive UChB and UChA rats. Environmental cues were paired with an intraperitoneal 50-mg/kg injection of acetaldehyde. On 10 consecutive days, each rat received one place conditioning per day; the acetaldehyde-pairing was alternated with saline-pairing. Results showed that conditioning with the 50-mg/kg dose of acetaldehyde induced place preference in UChB rats and place aversion in UChA rats. In a second experiment, UChB and UChA rats, pretested for ethanol preference, were injected with one 50-mg/kg dose of acetaldehyde or saline and tested for their voluntary ethanol consumption during 4 weeks. Results showed that the acetaldehyde dose induced a persistent and long-lasting enhancement of ethanol intake in UChB rats, but not in UChA rats. These results, together with the finding that after administration of a 50-mg/kg dose of acetaldehyde cerebral venous blood acetaldehyde levels in UChA rats were consistently higher than levels in UChB rats, support the suggestion that differential acetaldehyde levels, differential brain ALDH2 activity, or both were responsible for the different effects of acetaldehyde in the two rat lines.     

Role of context in ethanol tolerance and subsequent hedonic effects.

Two groups of male Sprague-Dawley rats received ethanol dose-effect tests for FR30, food-reinforced operant performance, in each of two environmental contexts, before and after a period of daily presession ethanol or saline injections. During the latter period, context alternated daily. The ethanol group received ethanol prior to sessions for one context and saline, prior to sessions for the other context. The saline group always received presession saline. The ethanol, but not the saline, group displayed robust tolerance to ethanol's rate-decreasing effects, with no difference between tests in each context. Both groups then received training and testing in an ethanol-conditioned place preference task. The saline group displayed significant avoidance of the compartment paired with ethanol. The ethanol group displayed no initial aversion for the ethanol compartment and, with extended conditioning, showed a significant increase in time spent in the ethanol compartment. We suggest that this tolerance represents context-independent, learning to compensate for ethanol-induced effects, and that this tolerance subsequently blocked the conditioned place aversion evident in nontolerant controls, thereby enhancing the estimates of ethanol's reward properties.     

Ethanol induces locomotor activating effects in preweanling Sprague-Dawley rats

Abuse of drugs exerts biphasic motor activity effects, which seem to be associated with their motivational effects. In the case of ethanol, heterogenous rat strains appear to be particularly sensitive to the sedative effects of the drug. In contrast, ethanol's activating effects have been consistently reported in rats genetically selected for ethanol affinity. Heightened ethanol affinity and sensitivity to ethanol's reinforcement are also observed in nonselected rats during early ontogeny. In the present study, we examined psychomotor effects of ethanol (1.25 and 2.5 g/kg) in 8-, 12-, and 15-day-old pups. Motor activity in a novel environment was assessed 5-10 or 15-20 min following drug treatment. Rectal temperatures and latency to exhibit the righting reflex were recorded immediately after locomotor activity assessment. Ethanol exerted clear activating effects at 8 and 12 days of age (Experiments 1a and 1b) and to a lesser extent at 15 days. At this age, ethanol enhanced locomotor activity in the first testing interval (Experiment 1b) and suppressed locomotion at 15-20 min (Experiment 1a). Ethanol-mediated motor impairment was more pronounced in the youngest group (postnatal day 8) than in the older ones. Blood ethanol concentrations were equivalent in all age groups. The present study indicates that preweanling rats are sensitive to ethanol's stimulating effects during the second postnatal week, and suggest that specific periods during early ontogeny of the rat can provide a valuable framework for the study of mechanisms underlying ethanol's stimulation and reinforcement effects.     

Ethanol induces second-order aversive conditioning in adolescent and adult rats

Alcohol abuse and dependence are considered public health problems, with an etiological onset often occurring during late childhood and adolescence, and understanding age-related differences in ethanol sensitivity is important. Low to moderate ethanol doses (0.5 and 2.0 g/kg, intragastrically [i.g.]) induce single-trial, appetitive second-order place conditioning (SOC) in adolescent, but not adult, rats. Recent studies have demonstrated that adolescents may be less sensitive than adults to the aversive properties of ethanol, reflected by conditioned taste aversion. The present study assessed the aversive motivational effects of high-dose ethanol (3.0 and 3.25 g/kg, i.g., for adolescents and adults, respectively) using SOC. Experiment 1 revealed similar blood and brain ethanol levels in adolescent and adult rats given 3.0 and 3.25 g/kg ethanol, respectively. In Experiment 2, animals received ethanol or vehicle paired with intraoral pulses of sucrose (conditioned stimulus 1 [CS1]). After one, two, or three conditioning trials, the rats were presented with the CS1 while in a distinctive chamber (CS2). When tested for CS2 preference, ethanol-treated animals exhibited reduced preference for the CS2 compared with controls. This result, indicative of ethanol-mediated aversive place conditioning, was similar for adolescents and adults; for females and males; and after one, two, or three training trials. In conjunction with previous results, the present study showed that, in adolescent rats subjected to SOC, ethanol’s hedonic effects vary from appetitive to aversive as the ethanol dose increases. Adolescent and adult animals appear to perceive the postingestive effects of high-dose ethanol as similarly aversive when assessed by SOC.     

Place conditioning with ethanol in rats bred for high (UChB) and low (UChA) voluntary alcohol drinking

The main goal of this study was to investigate the ability of an ethanol dose (1 g/kg) administered intraperitoneally to induce conditioned place preference (CPP) and/or conditioned place aversion (CPA) in two lines of rats selectively bred for their high (UChB) or low (UChA) voluntary ethanol intake. It was found that five pairings with ethanol induced CPA in ethanol-naïve rats of both lines, but the magnitude of avoidance was lower in the UChB relative to the UChA rats, indicating that ethanol was less aversive to naïve rats bred for high alcohol drinking. After 2 months of high voluntary ethanol drinking (∼6–7 g/kg/day), in free choice between 10% ethanol and water, ethanol produced CPP in UChB rats, reflecting that ethanol had become rewarding to these rats. By contrast, the low voluntary ethanol intake (<1 g/kg/day) displayed by UChA rats preexposed for 2 months in free choice did not change ethanol-induced CPA. However, preexposure of UChA rats to forced ethanol drinking (∼5.7 g/kg/day) and the later inhibition of ethanol-derived acetaldehyde by 4-methylpyrazole (10 mg/kg intraperitoneal), an inhibitor of the enzyme alcohol dehydrogenase, not only increased their voluntary ethanol intake in free choice, but also had a facilitating effect on the development of CPP. Taken together, these results show that the expression of the reinforcing effects of ethanol required a period of voluntary ethanol intake in UChB rats, whereas in UChA rats, both prior exposure to forced ethanol drinking and reduction of high blood ethanol-derived acetaldehyde were required.     

The catalase inhibitor sodium azide reduces ethanol-induced locomotor activity.

The involvement of brain catalase in modulating the psychopharmacological effects of ethanol was investigated by examining ethanol-induced locomotor activity in sodium azide-treated mice. Mice were pretreated with i.p. injections of the catalase inhibitor sodium azide (5, 10, or 15 mg/kg) or saline. Following this treatment, animals received i.p. injections of ethanol (0.0, 1.6, 2.4, or 3.2 g/kg). Ten minutes after ethanol administration, locomotor activity was recorded during a 10-min testing period in open-field chambers. The time effect between the two treatments (0, 30, 60, or 90 min) was also evaluated. Results indicated that sodium azide alone did not change spontaneous locomotor activity. However, this catalase inhibitor significantly reduced ethanol-induced locomotor activity when it was injected simultaneously or 30 min before ethanol injections. Moreover, perfused brain homogenates of mice treated with sodium azide also showed a significant reduction of catalase activity. No differences in blood ethanol levels were observed between sodium azide and saline pretreated animals. Results of an additional experiment showed that sodium azide (10 mg/kg, at 30 min) did not produce an effect on d-amphetamine- (2 mg/kg) or tert-butanol- (0.5 g/kg) induced locomotor activities. A specific interaction between ethanol and sodium azide at the level of the central nervous system is suggested. These results provide further support for the involvement of brain catalase in ethanol-induced behavioral effects. They also support the notion that acetaldehyde may be produced directly in the brain by catalase and that it may be an important regulator of ethanol's locomotor effects.     

A two dimensional model of alcohol consumption: possible interaction of brain catalase and aldehyde dehydrogenase

The possible existence of a biological marker system mediating voluntary consumption of ethanol in rats has been examined in a series of studies. The working hypothesis underlying this research was that acetaldehyde, the primary metabolite of ethanol, mediates the positive reinforcing properties of ethanol and thus underlies the voluntary consumption of ethanol in both animals and humans. We further hypothesized that brain catalase and aldehyde dehydrogenase, the enzymes controlling the production and elimination of acetaldehyde in the brain, may represent a biological marker system underlying the affinity of the animals to consume ethanol. Data demonstrating that the activity levels of these enzymes are positively correlated with alcohol ingestion seems to suggest that it is likely that the enzyme activity can serve as a predictor of the propensity to drink alcohol. A predictive model is proposed which describes the modulation of voluntary ethanol intake through the activity of these enzymes and their role in determining rates of formation and degradation of acetaldehyde in the brain.     

Evidence for conditioned place preference to a moderate dose of ethanol in adult male Sprague–Dawley rats

The present series of experiments examined affective properties of a moderate dose of ethanol using the conditioned place preference (CPP) paradigm in ethanol-naïve, adult male Sprague–Dawley rats. The apparatus and the procedure used were both unbiased. In Experiment 1, rats were given four 30 min conditioning sessions with 1.5 g/kg ethanol (i.p.) or an equivalent volume of saline on the paired side. Animals were found to demonstrate CPP to the ethanol-paired side, an unexpected finding at this relatively high dose in rats. To replicate this finding, and to examine the possibility of non-associative conditioning, an unpaired control group was included in Experiment 2. Once again, rats showed a CPP to the side paired with ethanol relative to either control group. Given that testing in an unfamiliar environment typically results in elevated levels of anxiety and that animals in Experiments 1 and 2 were not exposed to the apparatus prior to conditioning, Experiment 3 was conducted to examine the potential role of context unfamiliarity for induction of ethanol CPP in this test situation by varying whether animals were exposed to the apparatus prior to conditioning. In this study, pre-exposure to the CPP apparatus was found to eliminate the CPP to ethanol observed in rats who were not familiarized with the apparatus. Collectively, these studies demonstrate that ethanol-naïve rats can find ethanol reinforcing as indexed by the CPP test, and provide some evidence for the conditions under which this uncommon finding is observed.     

Conditioned place aversion to the "hangover" phase of acute ethanol administration in the rat.

The purpose of this study was to examine ethanol's delayed effects (termed hangover) using conditioned place testing. Four groups of rats received a single pairing of a distinctive environment (tactile and visual) 10 h after injection with ethanol (0, 2, 3, 4 g/kg, i.p. ) or saline in a counterbalanced design. Rats receiving 3 and 4 g/kg ethanol showed a conditioned place aversion to ethanol hangover. Conditioning 10 h after 0 or 2 g/kg ethanol did not produce a significant place preference or aversion. The results suggest that the hangover following an acute injection of high doses of ethanol (3-4 g/kg) produces a significant and dose-related conditioned place aversion in the rat.     

Autoshaping of ethanol drinking in rats: effects of ethanol concentration and trial spacing.

In two studies, we evaluated the effects of ethanol concentration and trial spacing on Pavlovian autoshaping of ethanol drinking in rats. In these studies, the brief insertion of an ethanol sipper conditioned stimulus (CS) was followed by the response-independent presentation of food unconditioned stimulus (US), inducing sipper CS-directed drinking conditioned responses (CRs) in all rats. In Experiment 1, the ethanol concentration in the sipper CS [0%-16% volume/volume (vol./vol.), in increments of 1%] was systematically increased within subjects across autoshaping sessions. Groups of rats received sipper CS-food US pairings (Paired/Ethanol), a CS-US random procedure (Random/Ethanol), or water sipper CS paired with food US (Paired/Water). In Experiment 2, saccharin-fading procedures were used to initiate, in the Ethanol group, drinking of 6% (vol./vol.) ethanol in 0.1% saccharin or, in the Water group, drinking of tap water in 0.1% saccharin. After elimination of saccharin, and across days, the duration of access to the sipper CS during each autoshaping trial was increased (5, 10, 12.5, 15, 17.5, and 20 s), and subsequently, across days, the duration of the mean intertrial interval (ITI) was increased (60, 90, 120, and 150 s). In Experiment 1, Paired/Ethanol and Random/Ethanol groups showed higher intake of ethanol, in terms of grams per kilogram of body weight, at higher ethanol concentrations, with more ethanol intake recorded in the Paired/Ethanol group. In Experiment 2, the Ethanol group drank more than was consumed by the Water group, and, for both groups, fluid intake increased with longer ITIs. Results support the suggestion that autoshaping contributes to sipper CS-directed ethanol drinking.     

Ethanol-induced motor activity in normal and acatalasemic mice.

The role of brain catalase in modulating the psychopharmacological effects of ethanol was investigated by examining ethanol induced motor activity in normal, C3H-N, and a corresponding group of acatalasemic C3H-A, mice. Following administration of one of three doses of ethanol (0.8, 1.6, and 3.2 g/kg) or saline, mice were placed in open field chambers and locomotor and rearing activity was measured during a 10-min testing period. A significant increase in locomotor activity was recorded in both groups of mice at lower doses of ethanol, while the higher dose produced a marked depression. Normal mice demonstrated more locomotor activity than acatalasemic mice at all ethanol doses. No differences between both groups of mice were observed in rearing activity. Also, no differences in blood ethanol levels were observed between the two substrains. Brain and liver residual catalase activity in the acatalasemic mice was found to be 40% and 50%, respectively, of normal mice. Furthermore, evidence for possible involvement of the peroxidatic activity in ethanol-induced motor activity is presented. These results suggest a role for centrally formed acetaldehyde as a factor mediating some of ethanol's psychopharmacological effects.     

The effects of aminotriazole and acetaldehyde on an ethanol drug discrimination with a conditioned taste aversion procedure.

The present study was designed to investigate whether acetaldehyde shares stimulus properties with ethanol using the conditioned taste aversion (CTA) baseline of drug discrimination learning. Animals were trained to discriminate ethanol (0.8 g/kg, i.p.) from saline using 11 consecutive cycles consisting of a pairing day and three nonpairing days. On pairing days, all animals were injected with ethanol 30 min prior to a 20-min limited access to a saccharin solution (0.1% w/v) and then immediately injected with either LiCl (0.15 M, 1.8 meq) or distilled water. On the three following nonpairing days, animals were injected with saline and 30 min later presented with the same saccharin solution for 20 min. No injections followed on these nonpairing days. Results showed that animals acquired discriminative stimulus control for ethanol after seven pairings. Pretreatment with the catalase inhibitor did not alter the discriminative control for ethanol. Generalization tests revealed that acetaldehyde substituted for ethanol at a dose of 0.3 g/kg. The results of the present study suggest that catalase inhibition did not reverse or alter the discriminative stimulus effects of ethanol. However, generalization tests showed that acetaldehyde (0.3 g/kg) will substitute for ethanol suggesting that these two drugs share some similar properties.     

A multi-dimensional examination of the positive reinforcing properties of acetaldehyde

The suggestion that acetaldehyde may be endowed with positive reinforcing properties and may in fact mediate some of the psychopharmacological actions of ethanol has been examined by us and other investigators using a variety of paradigms. We first reported that non-dependent animals would self-administer acetaldehyde through an intra-cerebroventricular route. In addition, we have demonstrated that central infusion induced a conditioned place preference. We have also shown that an animal's propensity to self-administer acetaldehyde directly into the brain was related to its subsequent voluntary intake of ethanol. Lastly, we have reported that inhibition of acetaldehyde metabolism resulted in an enhancement of alcohol-induced euphoria in man. The data collected to date from four different paradigms strongly support the notion that acetaldehyde is endowed with positive reinforcing properties which may play a critical role in the mediation of ethanol euphoria.     

Prenatal ethanol increases sucrose reinforcement,an effect strengthened by postnatal association of ethanol and sucrose

Late prenatal exposure to ethanol recruits sensory processing of the drug and of its motivational properties, an experience that leads to heightened ethanol affinity. Recent studies indicate common sensory and neurobiological substrates between this drug and sweet tastants. Using a recently developed operant conditioning technique for infant rats, we examined the effects of prenatal ethanol history upon sucrose self-administration (postnatal days, PDs 14–17). Prior to the last conditioning session, a low (0.5 g/kg) or a high (2.5 g/kg) ethanol dose were paired with sucrose. The intention was to determine if ethanol would inflate or devalue the reinforcing capability of the tastant and if these effects are dependent upon prenatal ethanol history. Male and female pups prenatally exposed to ethanol (2.0 g/kg) responded more when reinforced with sucrose than pups lacking this antenatal experience. Independently of prenatal status, a low ethanol dose (0.5 g/kg) enhanced the reinforcing capability of sucrose while the highest dose (2.5 g/kg) seemed to ameliorate the motivational properties of the tastant. During extinction (PD 18), two factors were critical in determining persistence of responding despite reinforcement omission. Pups prenatally exposed to ethanol that subsequently experienced the low ethanol dose paired with sucrose, showed higher resistance to extinction. The effects here reported were not associated with differential blood alcohol levels across prenatal treatments. These results indicate that fetal ethanol experience promotes affinity for a natural sweet reinforcer and that low doses of ethanol are also capable of enhancing the positive motivational consequences of sucrose when ethanol and sucrose are paired during infancy.     

Prenatal ethanol exposure leads to greater ethanol-induced appetitive reinforcement

Prenatal ethanol significantly heightens later alcohol consumption, but the mechanisms that underlie this phenomenon are poorly understood. Little is known about the basis of ‘this effect of prenatal ethanol on the sensitivity to ethanol’s reinforcing effects. One possibility is that prenatal ethanol exposure makes subjects more sensitive to the appetitive effects of ethanol or less sensitive to ethanol’s aversive consequences. The present study assessed ethanol-induced second-order conditioned place preference (CPP) and aversion and ethanol-induced conditioned taste aversion (CTA) in infant rats prenatally exposed to ethanol (2.0 g/kg) or vehicle (water) or left untreated. The involvement of the κ opioid receptor system in ethanol-induced CTA was also explored. When place conditioning occurred during the ascending limb of the blood–ethanol curve (Experiment 1), the pups exposed to ethanol in utero exhibited greater CPP than untreated controls, with a shift to the right of the dose–response curve. Conditioning during a later phase of intoxication (30–45 min post-administration; Experiment 2) resulted in place aversion in control pups exposed to vehicle during late gestation but not in pups that were exposed to ethanol in utero. Ethanol induced a reliable and similar CTA (Experiment 3) in the pups treated with vehicle or ethanol during gestation, and CTA was insensitive to κ antagonism. These results suggest that brief exposure to a moderate ethanol dose during late gestation promotes ethanol-mediated reinforcement and alters the expression of conditioned aversion by ethanol. This shift in the motivational reactivity to ethanol may be an underlying basis of the effect of prenatal ethanol on later ethanol acceptance.