Self-administration training

Following surgery, rats (n=10–15 per group at the start of each experiment) were trained to self-administer IV cocaine (0.25 mg/0.1ml/infusion; a generous gift from the National Institute on Drug Abuse, Bethesda, MD, USA) during daily 6-hr sessions on a FR1 schedule of reinforcement. At the start of each session, the rat’s catheter was connected to a mortorized pump (located outside of the sound attenuated chamber) via a liquid swivel as previously described e.g., [16–19, 25, 60]. Active lever-presses resulted in a 5-sec activation of the infusion pump and a 20-sec presentation of a stimulus complex, consisting of activation of the white stimulus light above the active lever and the tone generator (78 dB, 2 kHz), during which responses on the active lever had no consequences. Responses on the inactive lever were recorded, but had no programmed consequences. Rats were trained to self-administer cocaine for 10 sessions and over-dose was prevented by capping the number of cocaine infusions permitted during the first 2 days of training at 100 (day1) and 120 (day2). Rats failing to meet self-administration criterion (minimum of 50 infusions/6-h session for the last 3 days of training) were excluded from the study.

For the immunoblotting study, additional groups of rats were trained to self-administer IV saline (0.1 ml/infusion) during daily 1-h or 6-h sessions (n=12/control group at the start of the experiment) to provide a baseline for determination of cocaine-elicited changes in protein expression [18]. We included both Sal1h and Sal6h animals in our immunoblotting experiments to verify that the duration of the saline self-administration session does not significantly influence NMDA subunit expression of relevance to the design of the neuropharmacological study. As it did not, the neuropharmacological study included Sal1h animals, as well as, an additional group of rats trained to lever-press for 45 mg sucrose pellets (Noyes) during daily 6-h sessions, to examine for the reinforcer-specificity of the effects of intracranial ifenprodil upon cue-elicited behavior. All self-administration training and testing occurred during the dark phase of the circadian cycle. Upon completion of the 10 days of self-administration training, animals remained in their home cages in the colony room until testing for cue-elicited lever-pressing behavior or tissue collection at either 3 or 30 days following the last self-administration session.