Monkeys trained to self-administer nicotine (30 μg/kg/injection, i.v.), as described above, were placed under extinction by substituting vehicle for nicotine but maintaining the response-dependent presentation of the nicotine-paired stimulus. When responding reached a low, stable level (<10 injections/session, with no obvious increasing or decreasing trend), priming injections (vehicle or 0.1 mg/kg i.v. nicotine, immediately before the session) were given after pretreatment with WY14643 (20 or 40 mg/kg) or WY14643 (20 or 40 mg/kg) plus MK886 (1 mg/kg). Pretreatments were given in the home cage, and primes were given while the monkey was in the training chamber. Each test was performed for a single session followed by extinction sessions with no pretreatment.
