Ethanol as an oral reinforcer in normal weight rhesus monkeys: dose-response functions

Responding maintained by orally delivered ethanol was studied in three adult male rhesus monkeys with no previous drug-taking histories. The subjects had continuous access to water throughout the study and were fed sufficient food to maintain a positive caloric balance. During daily 3-h experimental sessions, ethanol solution and water were concurrently available under fixed-ratio reinforcement schedules from two liquid delivery systems that were mounted symmetrically on the side of the monkeys' cages. The positions of the ethanol and water alternated daily. A range of ethanol doses (1%, 2%, 4%, 5.7%, 8%, 11.3%, 16%, 22.7, and 32% w/v) was tested in ascending and descending order. Changes in concentration were made after six consecutive stable sessions. Generally, the number of ethanol deliveries increased and then decreased as the ethanol concentration was raised (an inverted U-shaped function). For two of the monkeys, ethanol deliveries exceeded water deliveries at concentrations from 1.0% to 5.7% w/v, whereas for a third monkey ethanol solution was preferred to water at concentrations as high as 16% w/v. Ethanol intake (g/kg) tended to increase linearly and then flatten as the concentration was raised. Blood ethanol levels, measured immediately following sessions in which 8% w/v ethanol was available, ranged from 45 to 146 mg/dl. Although no special training or acquisition procedures were used to establish responding, ethanol functioned as a reinforcer for each of the monkeys tested.