Differential reinforcement and diurnal rhythms of lever pressing for ethanol in AA and Wistar rats

High-drinking AA (Alko, Alcohol) and moderate-drinking Wistar rats, after ethanol drinking experience in their home cages, were housed continually in operant chambers with free access to water and food. Ethanol and water could be obtained by lever pressing on a concurrent FR1:FR1 schedule. The AA rats readily learned the operant response for oral ethanol, responded significantly more for ethanol than water, and increased ethanol responding when the fixed-ratio schedule for it was increased from FR1 to FR2 and FR4. This indicates that ethanol was serving as a reinforcer for the AAs. In contrast, the Wistars showed little evidence for ethanol reinforcement. Both AAs and Wistars had a three-peak pattern of ethanol responding during the dark phase, but peaks for the Wistars preceded those for the AAs by 1 or 2 hr. The patterns were similar when on an FR4 schedule, which greatly reduced the amount of alcohol, suggesting that they are not controlled by blood alcohol levels. The difference between the AA and Wistar patterns may, however, be related to the differential ethanol reinforcement.