Maintenance of schedule-controlled behavior by intravenous injections of nicotine in squirrel monkeys

Lever pressing by squirrel monkeys was maintained by i.v. injections of nicotine (3-560 ig/kg) or cocaine (3-300 pig/kg) under two intermittent schedules of self-administration. Under a fixed-interval schedule, the first response after a specified interval of time produced an injection. Under a second-order fixed-interval schedule, the completion of every 1 0-response fixed-ratio unit produced a brief visual stimulus and the first fixed-ratio unit completed after a specified interval produced both the brief stimulus and an injection. As the dose of either drug was increased, the rate of responding first increased and then decreased; maximal response rates maintained by nico-tine were approximately equal to those maintained by cocaine in some monkeys, but less than those maintained by cocaine in other monkeys. Patterns of responding maintained by the two drugs were qualitatively similar in all monkeys and wer