Ethanol drinking patterns in a continuous-access operant situation: effects of ethanol concentration and response requirements.

Rats, initiated to self-administer ethanol with either a sucrose-substitution procedure or a secondary-conditioning procedure, were maintained in a continuous-access environmental system in which operant lever press responses were required to receive 10% ethanol and food reinforcement. Water available from a drinking tube was electronically monitored to detect licks. Total daily consumption and patterns of food, water, and ethanol responding were analyzed under conditions in which the concentration of ethanol presented as a reinforcer was either 10% or 20%, and the response requirement for ethanol reinforcement was either a fixed ratio 4 schedule or a fixed ratio 1 schedule. Either increasing the ethanol concentration or decreasing the response requirement resulted in an increase in total daily ethanol intake. There was no significant difference between initiation procedures. These results are similar to observations in studies using a limited-access operant situation. This increased ethanol intake resulted from a complex alteration in the daily ethanol drinking pattern. The greatest ethanol intakes were observed when both the ethanol concentration was increased and response requirement was decreased. This was predominantly the result of increasing the number of ethanol drinking bouts per day when the response requirement was decreased, and by decreasing individual bout size by less than half when the ethanol concentration was doubled. These studies indicate that concentration of the ethanol presented as the reinforcer and the response cost required for reinforcement are involved in regulating ethanol consumption in the continuous-access condition. Type of initiation did not appear to interact with these variables.