Fig. 3.

Effects of cocaine (a) and amphetamine (b) on extracellular levels of dopamine before drug self-administration (drug-naïve) and following 10 weeks of limited and extended access (n=3). Dopamine levels are expressed as the mean (±SEM) percentage of baseline levels during 10-min sampling intervals beginning 1 h after probe implantation. Dashed vertical line at time point 0-min represents administration of drug after collection of six baseline samples. The peak percent increase in dopamine in response to both doses of cocaine and amphetamine is indicated for each drug access condition. The asterisk indicates significant difference from the drug-naïve state at time point 20-min