TOLERANCE TO EFFECTS OF COCAINE ON BEHAVIOR UNDER A RESPONSE-INITIATED FIXED-INTERVAL SCHEDULE

Tolerance to effects of cocaine can be modulated by schedules of reinforcement. With multiple ratio
schedules, research has shown an inverse relationship between ratio requirement and amount of
tolerance that resulted from daily administration of the drug. In contrast, tolerance to the effects of
cocaine on behavior under multiple interval schedules generally has developed regardless of interval
value. Under interval schedules reinforcement depends on the animal making one response following a
time interval. Thus, as time to respond increases, the time to reinforcement decreases. On the other
hand, fixed ratio schedules require a specified number of responses to be made prior to reinforcement.
Therefore, delaying the initiation of responding does not coincide with a significant decrease in the
time to reinforcement. In the current experiment, 6 pigeons were trained to respond under a threecomponent
multiple schedule, with a different tandem fixed-ratio 1 fixed-interval schedule in each
component. The multiple schedule required one response, which was followed by one of three fixedinterval
values (5, 15, or 60 s). Thus, the multiple schedule was interval-like because after the fixed-ratio
1, only one more response was required for reinforcement, but it was also ratio-like because the length
of the pause at the beginning of each interreinforcer interval affected the time until the next reinforcer.
Acute administration of cocaine generally resulted in dose-dependent decreases in responding. Chronic
(i.e., daily) administration of a rate-decreasing dose resulted in tolerance patterns similar to those
usually obtained with multiple ratio schedules. That is, the magnitude of tolerance was related inversely
to schedule size. These results suggest that delay to reinforcement from the initial response may play a
role in the development of schedule-parameter-related tolerance.