General procedure and apparatus described previously (9,19). Self-administration sessions (2 hr/day) under a one-response fixed-ratio (FR1) schedule of i.v. nicotine injection (10 or 30 μg/kg/injection) began 7-10 days after catheterization, and increased to FR5 over 15-23 sessions. Responses in the left nose-poke hole produced nicotine and pulsed the houselight (5 Hz) for a 20 s timeout. Responses in the other, inactive, hole had no scheduled effect. Once a criterion was reached (>9 nicotine injections/session for three consecutive sessions), rats received a pretreatment injection (either drug or vehicle) before each subsequent session, with drugs tested only when the criterion had been met during the preceding three vehicle sessions. Doses of WY14643 (20 or 40 mg/kg i.p., 20 min before the session, tested in counterbalanced order) or methOEA (10 mg/kg i.m., 40 min before the session) were each tested for three consecutive sessions. Mean data from the three drug pretreatment sessions for each dose were compared to the mean from the three preceding vehicle pretreatment sessions.
