Fig. 1.

Drug-taking behavior over the course of weeks. (A) Depiction of a rat connected to voltammetric recording equipment and infusion pump for i.v. delivery of cocaine during an approach to the active nose-poke port in the operant chamber. (B) A nose poke (dashed line) into the active port elicits an infusion of cocaine (0.5 mg/kg per infusion) and the presentation of a CS (yellow box) during a 20-s time-out. (C) Nose pokes into the active port, inactive port, and during the time-out period over 20 d of self-administration (n = 18). (D) The number of reinforced nose pokes did not change significantly across weeks, whereas the number of nonreinforced responses decreased (E), and the ratio of reinforced over total number of nose pokes (efficiency) increased (F) in the second and third weeks compared with the first week. *P < 0.05, ***P < 0.001; n.s., not significant.