Nicotine self-administration in baboons.

Two experiments were conducted in which responding maintained by nicotine and cocaine was studied under two different schedules of drug delivery. In Experiment 1, nicotine (0.01-0.32 mg/kg IV) was available under a fixed-ratio 2 timeout 15 sec reinforcement schedule. When nicotine was substituted for cocaine or saline, dose-dependent differences in self-administration were evident across the first five sessions, resulting in an inverted U-shaped dose-effect curve. With continued exposure to each nicotine dose, however, number of injections generally stabilized at levels not very different, if at all, from saline; and the terminal dose-effect functions generally were low and flat. In Experiment 2, nicotine (0.01-0.56 mg/kg IV) was available under a fixed-interval 5 min timeout 60 sec reinforcement schedule. Response rates were considerably lower and response patterning was less likely to be scalloped than when responding was maintained by either cocaine or food, but number of injections was higher than those maintained by saline. When fixed-interval value was varied, number of nicotine reinforcements remained low and virtually constant, but number of food reinforcements increased as the fixed interval decreased. The present results, along with those from previous studies, suggest that the ability of nicotine to serve as a reinforcer appears to be strongly influenced by the conditions of drug availability, perhaps more so than for other drugs of abuse.