Methohexital and cocaine self-administration under fixed-ratio and second-order schedules

Behavior maintained by either cocaine or methohexital was compared under two different schedules of drug delivery. Under a fixed-ratio 10 schedule, each tenth response produced an injection and responding was characterized by pauses alternating with high rates that were sustained until the drug injection. Under a second-order schedule, each tenth response produced a brief visual stimulus, and the first sequence of ten responses emitted after the lapse of a ten-minute interval produced the stimulus and the drug injection. Responding under the second-order schedule was characterized by an overall positive acceleration in responding that consisted of fixed-ratio response patterns terminating in the presentation of a brief stimulus. Under either schedule, each drug maintained maximal rates of responding at intermediate doses. In most respects, rates and patterns of responding depended more on the schedule of drug delivery than on the particular drug maintaining responding.