Prolonged effects of repeated cocaine on medial prefrontal cortex dopamine response to cocaine and a stressful predatory odor challenge in rats.

The present study examined the effects of seven daily saline (1 ml/kg, i.p.) or cocaine injections (15 mg/kg, i.p.) on extracellular dopamine levels in the medial prefrontal cortex (mPFC) after challenge with cocaine or stressful predatory odor presentation given 1 week (early withdrawal) or 3 weeks later (late withdrawal). Cocaine challenge at early withdrawal produced an increase in dopamine levels that was temporally shifted so that maximal levels of dopamine were significantly higher and attained 20 min earlier in the cocaine-pretreated group (maximal levels of saline controls=378% increase, cocaine=494% increase above baseline). Cocaine challenge at late withdrawal produced a similar effect on the temporal shift of maximal dopamine levels, with a significantly higher maximal percent increase of dopamine in cocaine-pretreated rats (saline-pretreated=420% increase, cocaine-pretreated=515% increase). Challenge with TMT, a predatory odor from fox that produces a stress response in rats, produced a maximal 75-200% increase in basal dopamine levels in both groups at both early and late withdrawal times. As with cocaine challenge, daily cocaine produced a leftward shift in the time at which maximal dopamine levels were attained in response to TMT. Cocaine-pretreated animals demonstrated maximal dopamine levels 40-80 min after TMT removal, while saline-pretreated rats showed maximal levels 100-140 min after TMT removal. These results suggest that there are long-term changes in the mPFC dopamine response to subsequent challenge with cocaine as well as a stressful predatory odor. The altered response of mPFC dopamine after repeated daily cocaine may impact relapse to drug-seeking or drug-taking behavior.