Ethanol-induced conditioned taste aversion to ethanol

Binge ethanol exposure in late gestation induces ethanol aversion in the dam but enhances ethanol intake in the offspring and affects their postnatal learning about ethanol

Previous studies show that exposure to 1 or 2g/kg of ethanol during the last days of gestation increases ethanol acceptance in infant rats. We tested whether prenatal exposure to 3g/kg, a relatively high ethanol dose, generates an aversion to ethanol in both the dam and offspring, and whether this prenatal experience affects the expression of learning derived from ethanol exposure postnatally. The answer was uncertain, because postnatal administration of a 3-g/kg ethanol dose induces an aversion to ethanol after postnatal day (PD) 10 but increases ethanol acceptance when administered during the first postnatal week. In the present study, pregnant rats received intragastric administrations of water or ethanol (3g/kg) on gestation days 17-20. On PDs 7-8 or 10-11, the offspring were administered water or ethanol (3g/kg). Intake of ethanol and water, locomotor activity in an open field, and ethanol odor preference were evaluated in the pups, whereas the mothers were evaluated in terms of ethanol intake. Results indicated an aversion to ethanol in dams that had been administered ethanol during gestation, despite a general increase in ethanol intake observed in their pups relative to controls. The prenatal ethanol exposure also potentiated the increase in ethanol intake observed after intoxication on PDs 7-8. Ethanol intoxication on PDs 10-11 reduced ethanol consumption; this ethanol aversion was still evident in infant rats exposed prenatally to ethanol despite their general increase in ethanol intake. No effects of prenatal ethanol exposure were observed in terms of motor activity or odor preference. It is concluded that prenatal exposure to ethanol, even in a dose that induces ethanol aversion in the gestating dam, increases ethanol intake in infant rats and that this experience modulates age-related differences in subsequent postnatal learning about ethanol.

Effect of pattern and number of chronic ethanol exposures on subsequent voluntary ethanol intake in C57BL/6J mice

RATIONALE

We previously demonstrated that chronic ethanol exposure and withdrawal experience significantly increased subsequent voluntary ethanol intake in C57BL/6J mice. This study was conducted to examine chronic ethanol conditions that optimize this enhanced ethanol-drinking behavior.

OBJECTIVES

The purpose of this study was to examine whether the pattern and/or number of chronic ethanol exposures influence subsequent ethanol intake.

METHODS

C57BL/6J mice were trained to drink ethanol (15% v/v) in a limited access situation (2 h/day) until stable intake was achieved. In experiment 1, mice received two cycles of chronic ethanol exposure delivered either in an intermittent [multiple withdrawal (MW)] or continuous [continuous exposure (CE)] pattern. One week of daily drinking sessions followed each exposure. In experiment 2, mice received either two or four cycles of chronic intermittent ethanol exposure (MW), each followed by a week of testing sessions. Three additional weeks of ethanol intake testing followed the last ethanol (or air) exposure.

RESULTS

Experiment 1: Only the MW group evidenced a significant increase in ethanol intake compared to controls after the first chronic ethanol exposure. Both MW and CE groups consumed more ethanol than controls after the second ethanol-exposure period. Experiment 2: Ethanol intake in MW mice compared to controls significantly increased after two or four cycles of chronic ethanol exposure/withdrawal, and this heightened ethanol intake lasted longer in mice that received four cycles of chronic intermittent ethanol exposure.

CONCLUSIONS

Increased voluntary ethanol intake after chronic ethanol exposure and withdrawal experience may be accelerated by intermittent (as opposed to continuous) ethanol exposure, and the effect may last longer with increased number of such experiences.

Flavor preferences conditioned by intragastric ethanol with limited access training

In a prior study, ad libitum fed rats learned a strong preference (90%) for a flavored saccharin solution (conditioned stimulus, CS+) paired with concurrent intragastric (IG) infusions of 5% ethanol over another flavor (CS-) paired with water infusions in unlimited access sessions (22 h/day). The present study expanded the investigation of ethanol-conditioned preferences to limited access sessions (30 min/day). Experiment 1 revealed that ad lib or food-restricted rats failed to develop a CS+ preference using the same CS solutions (0.05% Kool-Aid+0.2% saccharin) and IG infusions that were effective with long-term training. Experiments 2 and 3 mimicked the parameters from a report of successful ethanol conditioning in deprived rats: ethanol (0.5 g/kg) or water was infused intragastrically 5 min before access to sweetened CS solutions flavored with HCl or NaCl. Rats learned to prefer the ethanol-paired CS+ when the flavors were mixed with 5% sucrose but not when mixed with 0.2% saccharin. Experiment 4 revealed that 5% sucrose solutions flavored with 0.25% Kool Aid also supported flavor preference conditioning by IG ethanol (0.5 g/kg). CS+ preferences were obtained in rats trained with ethanol infused 5 min before or concurrent with CS+ intake, but not in rats trained with ethanol infused 30 min before CS+ intake. These data confirm that flavor preferences can be conditioned by IG ethanol using a limited access procedure. However, in contrast to 22 h/day training, 30 min/day training requires more intense CS flavors and a nutritive sweetener. The preference reinforcing actions of ethanol may develop slowly and are thus most effective with long training sessions or when intense CS flavors are used in short training sessions.

Flavor quality and ethanol concentration affect ethanol-conditioned flavor preferences

A previous report showed that outbred rats acquired preferences for a sweetened conditioned stimulus (CS) flavor paired with intragastric ethanol. To evaluate the role of sweet taste in ethanol conditioning, this study compared training with sweetened and unsweetened flavors. In Experiment 1, nondeprived rats were trained to drink one flavored solution (CS+, e.g., grape) paired with intragastric infusion of 5% ethanol and another (CS-, e.g., cherry) paired with intragastric water on alternate days. The volume of ethanol solution infused was matched to the volume of flavored solution the rats consumed. The sweet group's flavors initially contained 0.2% saccharin, reduced to 0.1%, 0.05%, and 0% over days; the plain group's flavors were unsweetened. The sweet group drank more and self-infused more ethanol during training and its preference for the CS+ over the CS- (without saccharin) exceeded that of the plain group (75% versus 62%). Experiment 2 equated total ethanol intake in rats trained with two combinations of flavor quality and ethanol concentration. The Sweet5 group drank flavors with 0.2% saccharin throughout training and tests and received 5% ethanol when they drank CS+, while the Plain10 group drank unsweetened flavors and the CS+ was paired with 10% ethanol. Despite equal daily ethanol doses, the Sweet5 group strongly preferred the CS+ (89%) while the Plain10 group avoided it (31%). The two groups continued to show opposite CS+ preference profiles even when both were tested with sweet CS flavors and 10% ethanol infusions. Thus, sweet taste contributes to the development of ethanol-conditioned flavor preferences, and this effect is not explained by a simple enhancement of ethanol intake.

Flavor preferences conditioned by intragastric infusion of ethanol in rats

Sprague-Dawley rats were trained 22 h/day to associate a flavored solution [conditioned stimulus (CS+)] with intragastric infusions of 6% ethanol and another flavored solution (CS-) with water infusions. The infusions were matched to the CS intakes so that the animals determined their timing and size. In Phase 1, chow and water were available ad libitum, and both CS flavors were initially sweetened with saccharin that was then faded out. The rats displayed a preference for the CS+ over the CS- under both reinforced and extinction conditions. When food-restricted in Phase 2, the rats displayed an increased preference for the CS+. In Phase 3, the rats were fed ad libitum chow and given preference tests with the CS+ paired with ethanol infusions of increasing concentration (6%, 12%, 18%, and 24%). Their preference for the CS+ over the CS- persisted, and self-administered ethanol dose increased with concentration to 5 g/kg/day. The ethanol-based conditioned flavor preference resembled those conditioned by carbohydrate and fat infusions, suggesting that at least some of reinforcing ability of ethanol may be related to its postingestive nutritive effects.

Conditioned taste aversions induced by alcohol and lithium in rats selectively bred for ethanol neurosensitivity

Rats that were selectively bred for differences in alcohol-induced sleep time (alcohol neurosensitivity) were tested for differences in formation and extinction of alcohol- and LiCl-induced conditioned taste aversions. Male rats bred for high, control, or low alcohol sensitivity (HAS, CAS, and LAS rats, respectively) were deprived of water and given daily 30 min access to water for a baseline period of 7 days. Rats were then given a novel 0.125% sodium saccharin solution, followed by an intraperitoneal injection of either saline, 2 g/kg of ethanol (at 10% w/v), or 50.9 mg/kg of LiCl (0.15 M) on 3 conditioning days. Each saccharin exposure was followed by a recovery day of access to water. The ethanol-induced saccharin aversion extinguished more rapidly in LAS rats than in CAS or HAS rats (p < 0.05), but LiCl conditioned equivalent aversions in each group. Also, ethanol injection results in large differences in observed resting behavior in these rats (HAS > CAS > LAS), but LiCl injection produced no reliable group differences in resting. The weaker alcohol-induced taste aversion in LAS rats accords with their previously measured higher oral consumption of alcohol (Kulkosky et al., Alcoholism 17:545-551, 1993) and the idea that alcohol intake is limited by an expectancy of postingestive consequences. The weaker ethanol-induced aversion in LAS rats reflects selective breeding of an alcohol-specific trait and not a general difference in aversive conditioning or chemical neurosensitivity.

Animal learning and motivation and addictive drugs

Highlights of a systematic analysis of the abstracts of over 1700 publications dealing with addictive drugs (primarily alcohol) in the context of animal learning and motivation are summarized under two main headings. The behavioral effects of drugs vary with the nature of the drug, the dosage, and the behavioral baseline; behavioral tolerance frequently results from continued practice in the drug state. The paradigmatic effects show that drugs can function effectively as conditional stimuli, unconditional stimuli, responses, and reinforcers. As a result, drug habits develop their own motivational support, leading to conditioned tolerance and conditioned addiction. It is contended that principles of animal behavior can provide a basis for a theory of human drug use and abuse, but that voluntary control of addictive behavior requires uniquely human cognitive processes.

The AA and ANA rat lines, selected for differences in voluntary alcohol consumption

The offspring of rats that voluntarily select larger quantities of alcohol are heavier consumers of alcohol than the offspring of rats that tend to avoid it. Such selective breeding, repeated over many generations, was used to develop the AA (Alko, Alcohol) line of rats which prefer 10% alcohol to water, and the ANA (Alko, Non-Alcohol) line of rats which choose water to the virtual exclusion of alcohol. In addition to demonstrating the likely role of genetic factors in alcohol consumption, these lines have been used to find behavioral, metabolic, and neurochemical correlates of differential alcohol intake. Some of the line differences that have been found involve the reinforcing effects of ethanol, the changes in consumption produced by alcohol deprivation and nutritional factors, the behavioral and adrenal monoamine reactions to mild stress, the development of tolerance, the accumulation of acetaldehyde during ethanol metabolism, and the brain levels of serotonin. It is hoped that these studies will lead to a better understanding of the genetically-determined mechanisms that influence the selection of alcohol.

Differences in response to the aversive properties of ethanol in rats selectively bred for oral ethanol preference

A conditioned taste aversion (CTA) paradigm was used to determine whether aversion to the pharmacological effects of ethanol, apart from orosensory cues, can contribute to genetic differences in voluntary ethanol consumption. Four doses of ethanol, administered IP, were paired with the consumption of a 0.1% saccharin solution in rats from the alcohol-preferring (P) and alcohol-nonpreferring (NP) lines. Repeated pairing of saccharin and ethanol in a dose of 1.0 g/kg produced stronger and more prolonged aversion to saccharin in NP rats, compared with P rats, at comparable blood ethanol levels. A low dose of ethanol (0.25 g/kg) produced transient conditioned facilitation of saccharin consumption in P rats, but not in NP rats, at comparable blood ethanol levels. The results suggest that rats of the NP line find the postingestional effects of high-dose ethanol more aversive, and low-dose ethanol less reinforcing, than do rats of the P line. Genetic differences in voluntary ethanol consumption may be due, in part, to differences in aversion to the postingestional effects of ethanol.

Effect of taste aversion learning on ethanol self-administration

Ethanol (EtOH) oral self-administration studies using rats have had inconsistent outcomes: studies in which rats are fluid deprived report decreasing EtOH intake over trials, whereas studies not employing fluid deprivation report increasing intake over trials. The present study supports the hypothesis that differential taste aversion learning may account for some of this discrepancy. This study indicates that taste aversion learning is maximized under fluid deprivation conditions and that "latent inhibition," i.e., exposure to non-intoxicating amounts of the EtOH solution prior to conditioning, reduces taste aversion learning. It is suggested that the effect of fluid deprivation on taste aversion resulting from EtOH self-administration may be at least in part due to the development of latent inhibition in non-deprived animals during initial exposure to the EtOH solution.

Oral ethanol reinforcement in the rat: effect of the partial inverse benzodiazepine agonist RO15-4513

The partial inverse benzodiazepine agonist RO15-4513 has been found to reverse the sedating and anti-conflict effects of acute ethanol administration. In non-food or fluid-deprived rats, orally self-administering 10% ethanol in an operant situation, RO15-4513 resulted in a dose-dependent suppression on ethanol intake. Doses of 0.3, 1.0 and 3.0 mg/kg suppressed responding from approximately 25% to 60% respectively. A dose of 0.1 mg/kg had no significant effect upon responding. These findings were discussed in terms of the potential independence of brain mechanisms related to ethanol reinforcement and sedation.

Cyanamide injections during alcohol deprivation increase alcohol drinking

Long-Evans male rats were given 7 weeks of choice between 10% ethanol and water and then were divided into 6 matched groups, 3 of which were then deprived of alcohol for 6 days. Subcutaneous cyanamide injections (10 mg/kg, 3 times daily, for 4 days) during alcohol deprivation produced a long lasting, significant increase in subsequent alcohol selection, over and above the increase produced by alcohol deprivation alone. The same injections given to a group not deprived of alcohol caused a significant suppression of alcohol drinking during the treatment and had disappeared 4 days after the last injection. Thereafter the drinking remained at the control level and did not rise to that of the group given the injections during deprivation. The groups did not differ in their subsequent selection of saline solutions.