Pentobarbital drinking by rhesus monkeys: establishment and maintenance of pentobarbital-reinforced behavior.

Orally delivered pentobarbital was established as a reinforcer for 5 rhesus monkeys. The monkeys were food deprived throughout the study. LiQUid deliveries (0. 5 ml) were contingent upon 1 ip-contact responses on a drinking spout, and food pellet deliveries were contingent upon lever-press responses. Three-hr daily sessions were preceded and followed by 1-hr blackouts. In the initial phase water was del i vered fo 11 owing each 1 i p-contact response both within the 3-hr sessions and between sessions. During the second hr of the daily 3-hr session, each lever press produced one food pellet until a limit (65 to 100 pellets) was reached. This procedure resulted in elevated levels of water drinking that varied among monkeys from 334 to 983 ml per session. Subsequently during sessions an increasing series of pentobarbital concentrations (from 0.0078 up to 1 mg/ml) was provided via the drinking spout while water was continuously available between sessions. Pentobarbital intake ranged from 12 to 45 mg/kg/3-hr session, and intoxication was noted when intake exceeded 20 mg/kg. In the next phase, the monkeys had 1 imi ted access to food after each session, continuous access to water between sessions, and access to either pentobarbital or water during the 3-hr sessions. Both the concentration of pentobarbital and the number of 1 ip-contact responses required per drug delivery were gradually increased until the pentobarbital concentration was 4.0 mg/ml and the response requirement was 16 (M-H and M-P1), 32 (M-BL), 45 (M-P), or 64 (M-W). During the phase when 4.0 mg/ml pentobarbital was replaced by water, liquid deliveries decreased to low levels. Liquid deliveries increased to previous levels when 4.0 mg/ml pentobarbital was reintroduced. When the pentobarbital concentration was decreased (4.0 mg/ml, 2.0, 1.0, • ) , pentobarbital intake decreased (mg/kg/3-hr session) and the number of liquid deliveries first increased and then decreased.