Figure 2.

Substitution of cocaine resulted in the acquisition of self-administration in rats that did not self-administer σ₁R agonists. (a, b) Acquisition of cocaine (0.32 mg/kg/injection) self-administration when each response produced an injection after extended exposure to PRE-084 (0.32 mg/kg/injection) or after exposure to (+)-pentazocine (0.32 mg/kg/injection). Each point represents the mean ±SEM of six subjects. ***p<0.001, compared with responding on the inactive lever (post-hoc Bonferroni t-test). (c) Examples of actual self-administration performances of a representative subject in real time. Ordinates, cumulative responses; abscissae, time. Each record is from a single 120-min experimental session. Each response on the active lever incrementally stepped the cumulative curve upward and produced a diagonal mark on the cumulative response curve. Vertical marks on the line below the cumulative curve indicate responses on the left (inactive) lever. The first record is from a previously naive subject in the last session with the opportunity to self-administer 0.32 mg/kg/injection of PRE-084. The second record is from that same subject after 14 sessions with the opportunity to self-administer 0.32 mg/kg/injection of cocaine. The third record is from the immediately following session, the first opportunity to self-administer 0.32 mg/kg/injection of PRE-084 after experience with cocaine. The last record shows stable self-administration of PRE-084.