The induction of oral ethanol self-administration by contingent ethanol delivery

Shortening time for access to alcohol drives up front-loading behavior, bringing consumption in male rats to the level of females

Background

Incentives to promote drinking (“happy hour”) can encourage faster rates of alcohol consumption, especially in women. Sex differences in drinking dynamics may underlie differential health vulnerabilities relating to alcohol in women versus men. Herein, we used operant procedures to model the happy hour effect and gain insight into the alcohol drinking dynamics of male and female rats.

Methods

Adult male and female Wistar rats underwent operant training to promote voluntary drinking of 10% (w/v) alcohol (8 rats/sex). We tested how drinking patterns changed after manipulating the effort required for alcohol (fixed ratio, FR), as well as the length of time in which rats had access to alcohol (self-administration session length). Rats were tested twice within the 12 h of the dark cycle, first at 2 h (early phase of the dark cycle, “early sessions”) and then again at 10 h into the dark cycle (late phase of the dark cycle, “late sessions”) with an 8-h break between the two sessions in the home cage.

Results

Adult females consumed significantly more alcohol (g/kg) than males in the 30-min sessions with the FR1 schedule of reinforcement when tested late in the dark cycle. Front-loading of alcohol was the primary factor driving higher consumption in females. Changing the schedule of reinforcement from FR1 to FR3 reduced total consumption. Notably, this manipulation had minimal effect on front-loading behavior in females, whereas front-loading behavior was significantly reduced in males when more effort was required to access alcohol. Compressing drinking access to 15 min to model a happy hour drove up front-loading behavior, generating alcohol drinking patterns in males that were similar to patterns in females (faster drinking and higher intake).

Conclusions

This strategy could be useful for exploring sex differences in the neural mechanisms underlying alcohol drinking and related health vulnerabilities. Our findings also highlight the importance of the time of testing for detecting sex differences in drinking behavior.

Voluntary alcohol drinking is higher in adult female rats compared to adult male rats. This sex difference is most pronounced in the later phase of the dark cycle, and when the operant effort is minimal (when 1 lever press gives 1 reward: fixed ratio 1, FR1).

Higher alcohol intake in females is primarily due to “front-loading”, or the rapid consumption of alcohol within the first 5 min of access.

Increasing the effort required to obtain alcohol from FR1 to FR3 dampens front-loading drinking behavior, resulting in similar levels of total intake in males and females.

Compressing the time of access to 15 min drives up front-loading to such a degree that rats end up consuming more alcohol in total than they do in 30-min sessions. In males, this increase in drinking is large enough that it eliminates the sex difference in total alcohol intake.

Effects of cannabidiol plus naltrexone on motivation and ethanol consumption

BACKGROUND AND PURPOSE

The aim of this study was to explore if the administration of naltrexone together with cannabidiol (CBD) may improve the efficacy in reducing alcohol consumption and motivation rather than any of the drugs given separately.

EXPERIMENTAL APPROACH

The effects of low doses of naltrexone (0.7 mg·kg^-1 , p.o.) and/or CBD (20 mg·kg^-1 ·day^-1 , s.c.) on ethanol consumption and motivation to drink were evaluated in the oral-ethanol self-administration paradigm in C57BL/6 mice. Gene expression analyses of the opioid μ receptor (Oprm1) in the nucleus accumbens (NAc), tyrosine hydroxylase (TH) in the ventral tegmental area (VTA) and the 5-HT_1A receptor in the dorsal raphe nucleus (DR) were carried out by real-time PCR. The role of 5-HT_1A receptors in the ethanol reduction induced by the administration of CBD + naltrexone was analysed by using the 5-HT_1A receptor antagonist WAY100635 (0.3 mg·kg^-1 , i.p.).

KEY RESULTS

The administration of CBD + naltrexone significantly reduced motivation and ethanol intake in the oral self-administration procedure in a greater proportion than the drugs given alone. Only the combination of both drugs significantly reduced Oprm1, TH and 5-HT_1A gene expressions in the NAc, VTA and DR respectively. Interestingly, the administration of WAY100635 significantly blocked the actions of CBD + naltrexone but had no effects by itself.

CONCLUSION AND IMPLICATIONS

The combination of low doses of CBD plus naltrexone were more effective than either CBD or naltrexone alone at reducing ethanol consumption and the motivation to drink. These effects appear to be mediated, at least in part, by 5-HT_1A receptors.

Mystic Acetaldehyde: The Never-Ending Story on Alcoholism

After decades of uncertainties and drawbacks, the study on the role and significance of acetaldehyde in the effects of ethanol seemed to have found its main paths. Accordingly, the effects of acetaldehyde, after its systemic or central administration and as obtained following ethanol metabolism, looked as they were extensively characterized. However, almost 5 years after this research appeared at its highest momentum, the investigations on this topic have been revitalized on at least three main directions: (1) the role and the behavioral significance of acetaldehyde in different phases of ethanol self-administration and in voluntary ethanol consumption; (2) the distinction, in the central effects of ethanol, between those arising from its non-metabolized fraction and those attributable to ethanol-derived acetaldehyde; and (3) the role of the acetaldehyde-dopamine condensation product, salsolinol. The present review article aims at presenting and discussing prospectively the most recent data accumulated following these three research pathways on this never-ending story in order to offer the most up-to-date synoptic critical view on such still unresolved and exciting topic.

Role of ethanol-derived acetaldehyde in operant oral self-administration of ethanol in rats

RATIONALE

The role of ethanol-derived acetaldehyde has not been examined yet on performance in a model of operant oral self-administration. However, previous studies reported that an acetaldehyde-sequestering agent, D-penicillamine (DP) and an inhibitor of catalase-mediated acetaldehyde production, 3-amino-1,2,4-triazole (3-AT) reduce voluntary ethanol consumption.

OBJECTIVES

The aim of our investigation was to evaluate the effects of DP and 3-AT on acquisition and maintenance of oral operant ethanol self-administration.

METHODS

Using operant chambers, rats learned to nose poke in order to receive ethanol solution (5-10 % v/v) under an FR1 schedule of reinforcement in which discrete light and tone cues were presented during ethanol delivery.

RESULTS

DP and 3-AT impair the acquisition of ethanol self-administration, whereas its maintenance is not affected neither by drug given alone for both 10 or 5 % ethanol nor by drugs association for 5 % ethanol. Moreover, when the concentration of ethanol was diminished from 10 to 5 %, rats increased the rate of self-administration behaviour.

CONCLUSIONS

These findings suggest that brain acetaldehyde plays a critical role during acquisition of operant self-administration in ethanol-naïve rats. In contrast, during the maintenance phase, acetaldehyde could contribute to ethanol self-administration by a combined mechanism: On one hand, its lack (by DP or 3-AT) might result in further ethanol-seeking and taking and, on the other, inhibition of ethanol metabolism (by 3-AT) might release an action of the un-metabolised fraction of ethanol that does not overall result in compromising maintenance of ethanol self-administration.

Stress-induced reinstatement of alcohol-seeking in rats is selectively suppressed by the neurokinin 1 (NK1) antagonist L822429

RATIONALE

Genetic inactivation or pharmacological antagonism of neurokinin 1 (NK1) receptors blocks morphine and alcohol reward in rodents, while NK1 antagonism decreases alcohol craving in humans. The role of the NK1 system for relapse-like behavior has not previously been examined.

OBJECTIVE

Divergence between human and rodent NK1 receptors has limited the utility of NK1 antagonists developed for the human receptor species for preclinical studies of addiction-related behaviors in rats. Here we used L822429, an NK1 antagonist specifically engineered to bind at high affinity to the rat receptor, to assess the effects of NK1 receptor antagonism on alcohol-seeking behaviors in rats.

METHODS

L822429 (15 and 30 mg/kg) was used to examine effects of NK1 receptor antagonism on operant self-administration of 10% alcohol in 30-min daily sessions, as well as intermittent footshock stress- and cue-induced reinstatement of alcohol-seeking after extinction of lever responding.

RESULTS

At the doses used, L822429 did not significantly affect alcohol self-administration or cue-induced reinstatement, but potently and dose dependently suppressed stress-induced reinstatement of alcohol seeking, with an essentially complete suppression at the highest dose. The effect of L822429 on stress-induced reinstatement was behaviorally specific. The drug had no effect on conditioned suppression of operant responding following fear conditioning, locomotor activity, or self-administration of a sucrose solution.

CONCLUSIONS

To the degree that the reinstatement model provides a model of drug relapse, the results provide support for NK1 antagonism as a promising mechanism for pharmacotherapy of alcoholism, acting through suppression of stress-induced craving and relapse.

Novel versus familiar ethanol: a comparison of aversive and rewarding properties

Rats were given a series of the conditioning trials during which sucrose solution was paired with ethanol which had previously been experienced on 0, 3, or 9 occasions. On each conditioning trial, the rats received a Taste Reactivity Trial, a Taste Avoidance Trial, and a Place-Conditioning Trial. After the rats had received 4 conditioning trials, they were given a Conditioned Place Preference Test, a Taste Reactivity (TR) Test, and a Conditioned Aversion Extinction Test. Only the group conditioned with novel ethanol demonstrated aversive TR responses, although all CS + groups eventually demonstrated suppressed ingestive TR responses and enhanced neutral TR responses. Familiarization attenuated, but did not eliminate, ethanol-induced CTAs. There was no evidence of place conditioning.

Analysis of spontaneous alcohol drinking in rhesus monkeys

This analysis aims at determining to what extent spontaneous alcohol drinking in adult male rhesus monkeys (Macaca mulatta) represents ethanol-directed behaviour. It is shown that in a condition of free access to an ethanol/water solution (2 percent v/v) and drinking water, alcohol drinking was initiated in all subjects (n = 4) within a few days, without any specific induction procedure. Relationship between drinking behaviour and ethanol concentrations was studied in 8 subjects by use of a concurrent 3-bottle-design. 2 bottles containing ethanol solution (concentrations 2.4; 4.8; 8.16; 16.32 percent v/v), 1 bottle contained drinking water. When ethanol concentrations in the solutions increased, consumption of ethanol solutions decreased, of drinking water increased, and of total water decreased. Net ethanol intake from a certain solution was influenced by its concentration and the concentration of the concurrently available solution. After an initial increase, total net ethanol intake remained relatively constant. Consumed amounts of ethanol (on the average 2-6 ml.kg-1 per day) could lead to notable blood ethanol levels. Drinking from ethanol solution was not just an alternative for ingesting water. The observed alcohol drinking is interpreted as resulting from a central reinforcement of ethanol intake and avoidance of negative, potentially harmful effects of ethanol.

Demonstration of lever pressing for oral ethanol by rats with no prior training or ethanol experience

Male rats of the alcohol-preferring AA line were placed in an operant conditioning chamber with one lever delivering 10% alcohol solution and a second giving water. Free food and water were also continually available in the chamber so the animals should not have been motivated to obtain alcohol for reasons of hunger or thirst. The rats had never had alcohol previously. No shaping was used. The rats simply lived for the next 2 weeks in the operant chamber. All of them eventually learned to work for alcohol. Ethanol responding was significantly higher than pressing for water throughout the second week: on the last day, all rats pressed more than 300 times for alcohol and less than 40 times for water, took in a mean of 5.3 +/- 0.2 g/kg of ethanol, and obtained 72% of their total fluid as earned ethanol solution despite the presence of free water. Their acquisition was, however, much slower than that observed in male AA rats that had previously had prolonged access to drinking alcohol in their home cages. Living continually in the operant chamber is thought probably to have been an important factor in enabling the naïve rats to learn to work for alcohol.

Haloperidol and apomorphine effects on ethanol reinforcement in free feeding rats

Free feeding male Long Evans rats were trained to lever press on a Fixed Ratio 8 schedule for 10% ethanol reinforcement. Mean ethanol intake in 30-minute sessions was 0.38 g/kg. Subcutaneous apomorphine (APO: 0.025 to 0.5 mg/kg) and haloperidol (HAL: 0.005 to 0.0625 mg/kg) administered 15 minutes before sessions dose-dependently reduced responding, but only APO reduced momentary response rates. Low doses of HAL reduced the effect of 0.3 but not 0.05 mg/kg APO. When the rats were food-restricted, control response rates decreased, and APO (0.025 and 0.05 mg/kg) had no further effect. Results were discussed in terms of dopamine involvement in the mechanism of ethanol reinforcement.