BEHAVIOR OF RATS UNDER FIXED CONSECUTIVE NUMBER SCHEDULES: EFFECTS OF DRUGS OF ABUSE

Four rats responded under a simple fixed consecutive number schedule in which eight or more
consecutive responses on the run lever, followed by a single response on the reinforcement lever,
produced the food reinforcer. Under this simple schedule, dose–response curves were determined
for diazepam, morphine, pentobarbital, and phencyclidine. The rats were then trained to respond
under a multiple fixed consecutive number schedule in which a discriminative stimulus signaled
when the response requirement on the run lever had been completed in one of the two fixed
consecutive number component schedules. Under control conditions, the percentage of reinforced
runs under the multiple-schedule component with the discriminative stimulus added was much higher
than the percentage of reinforced runs under the multiple-schedule component without the
discriminative stimulus. All of the drugs decreased the percentage of reinforced runs under each of
the fixed consecutive number schedules by increasing the conditional probability of short run
lengths. This effect was most consistently produced by morphine. The drugs produced few differences
in responding between the multiple fixed consecutive number components. Responding under
the simple fixed consecutive number schedule, however, was affected at lower doses of the drugs
than was responding under the same fixed consecutive number schedule when it was a component
of the multiple schedule. This result may be due to the difference in schedule context or, perhaps,
to the order of the experiments.