Orally delivered pentobarbital as a reinforcer for rhesus monkeys with concurrent access to water: effects of concentration, fixed-ratio size, and liquid positions.

The number of liquid deliveries and pattern of concurrent pentobarbital and water drinking were studied in three food deprived rhesus monkeys during daily 3-hr sessions. During the daily sessions, deliveries of approximately 0.6 ml of each liquid occurred under fixed-ratio (FR) schedules of lip contact responses. Between sessions water was freely available. Session drinking was studied as a function of pentobarbital concentration (1.0, 1.41, 2.0, and 4.0 mg/ml) and FR size (4, 8, 16 and 32 lip contacts per delivery). The number of drug deliveries decreased with increases in drug concentration. Drug intake ranged from 21 to 52 mg/kg of body wt./3-hr session. At all concentrations and FR values tested, the number of pentobarbital deliveries substantially exceeded the number of water deliveries. The positive reinforcing effect of the pentobarbital was indicated by a consistent choice of drug over water irrespective of the side position of pentobarbital and by higher rates of drug responding. Pentobarbital drinking occurred in a negatively accelerated pattern whereas water drinking did not have any consistent pattern. Marked intoxication followed bouts of drug drinking.