Operant drinking of alcohol on a rate-contingent ratio schedule of reinforcement

Abstract

Seven rats were trained to drink 10 per cent ethanol from a drinkometer in order to obtain a condensed milk reinforcement. Reinforcements were programmed on a Rate Contingent Ration schedule in which only responses (licks) occuring at a predetermined rate counted towards the required ratio of 64. All rats drank ethanol for milk reinforcement during 45 daily, hour-long sessions, despite the fact that no animal was food deprived. When 10 per cent ethanol was substituted for milk reinforcement, all rats continued to drink alcohol for alcohol for 10 days. However, six of the animals showed a significant decrease in the mean number of reinforcements earned per day. Milk reinforcements were made available for a subsequent period of 10 days, but only one of the seven animals earned as many reinforcements per day as during the initial training period. Alcohol was available in the home cage during each of these three conditions. During the final phase of the experiment, alcohol was removed from the home cage and in the operant conditioning sessions, alcohol was once again substituted for milk reinforcements. Although each animal persisted in drinking some alcohol for alcohol for a period of 20 days, the mean number of alcohol ‘reinforcements’ earned each day was not significantly different from the number earned when alcohol was also available in the home cage. These data indicate that despite prolonged exposure to alcohol, rats did not develop tolerance or dependence upon alcohol. Since tolerance and dependency are the major pharmacological criteria for defining addiction, we conclude that the drinking patterns observed in this experiment are best described as preference drinking.