Repeated Cocaine Modifies the Mechanism by which Amphetamine Releases Dopamine

Effects of L-type (100 μm verapamil and 10 μm diltiazem) and N-type (1 μmconotoxin) calcium channel antagonists on amphetamine-induced increases in extracellular dopamine in the nucleus accumbens of saline- and cocaine-pretreated rats. After 100 min of baseline, a calcium antagonist was incorporated into the dialysis buffer for 100 min. For the next 300 min of the experiment, three increasing doses of amphetamine (0.1–10 μm, 100 min each) were included in the dialysis buffer in the presence of the same calcium channel blocker. In all three experiments, there was a significant main effect of amphetamine dose [verapamil (V),F_(24,336) = 15.25, p < 0.0001; conotoxin (C), F_(24,384) = 19.21,p < 0.0001; diltiazem (D),F_(24,342) = 60.28, p < 0.0001]. There were no other significant main effects or interactions.

The effect of botulinum toxin B on amphetamine (top)- or GBR-12909 (bottom)-induced increases in extracellular dopamine in the nucleus accumbens of saline- and cocaine-pretreated rats. Botulinum toxin B (1.0 ng/μl, 0.3 μl) was unilaterally microinfused (via a 33 gauge cannula) into the nucleus accumbens through the microdialysis cannula 15 min before the placement of the microdialysis probe. For the amphetamine experiment, a two-way ANOVA revealed a significant main effect of amphetamine dose (F_(19,285) = 24.28, p < 0.0001) and a significant interaction between treatment groups and dose of amphetamine (F_(19,285) = 1.59,p = 0.058). For the GBR-12909 experiment, there was a significant main effect of GBR-12909 dose (F_(19,190) = 2.12, p= 0.0058); there were no other significant main effects or interactions. *Significant difference from saline (p < 0.05, Fisher’s LSD).

The effect of the CaM-KII blocker, KN-93, on amphetamine- or GBR-12909-induced increases in extracellular dopamine in the nucleus accumbens of saline- and cocaine-pretreated rats. Baseline samples were collected for 100 min, followed by 100 min of KN-93 (10 μm). For the remaining 300 min of the experiment, three increasing concentrations of amphetamine (0.1, 1.0, and 10 μm) or GBR-12909 (1, 10, and 100 μm) were incorporated into the dialysis buffer for 100 min each with 10 μm KN-93. For the amphetamine experiment, there was a significant main effect of amphetamine dose (F_(19,285) = 26.25; p < 0.0001), with no other significant treatment effects or interactions. For the GBR-12909 experiment, there was a significant main effect of GBR-12909 dose (F_(19,475) = 9.77;p < 0.0001), with no other significant treatment effects or interactions. The effects of the KN-93 control compound, KN-92, also were assessed on amphetamine-induced increases in extracellular dopamine in the nucleus accumbens of saline- and cocaine-pretreated rats. The design of the experiment was identical to the amphetamine experiment described above, except that KN-92 (10 μm) was substituted for KN-93. The analysis of this experiment revealed a significant main effect of amphetamine dose (F_(19,285) = 8.77; p < 0.0001) and a significant amphetamine × treatment interaction (F_(19,285) = 1.59; p = 0.05). *Significant difference from saline (p < 0.05, Fisher’s LSD).