Figure 1.

Mean dopamine and glutamate levels in 10-min microdialysis samples taken before, during, and after the final heroin self-administration day in animals that had received 14 previous days of self-administration training. Before the microdialysis tests, animals were housed and dialyzed at low rates in the test chamber overnight. On the test morning, the infusion rate was increased and baseline samples taken; then (at t=0), the response lever was inserted into the chamber and heroin became available for 4 h. The times of lever insertion and retraction are indicated by dashed vertical gray lines. Initial lever presses usually occurred within 3 min of lever insertion; the animals pressed at ∼20 min intervals thereafter, receiving ∼300 ng/h of heroin, by the end of the session. Traces a–f below the graph show the temporal pattern of responding for each animal. Vertical hash marks on each timeline indicate the times (relative to the graph above) of each earned injection for each animal.