Ultrahigh‐resolution MRI reveals structural brain differences in serotonin transporter knockout rats after sucrose and cocaine self‐administration

Excessive use of cocaine is known to induce changes in brain white and gray matter. It is unknown whether the extent of these changes is related to individual differences in vulnerability to cocaine addiction. One factor increasing vulnerability involves reduced expression of the serotonin transporter (5‐HTT). Human studies have shown that inherited 5‐HTT downregulation is associated with structural changes in the brain. These genotype‐related structural changes may contribute to risk for cocaine addiction. Here, we tested this idea by using ultrahigh‐resolution structural magnetic resonance imaging (MRI) on postmortem tissue of 5‐HTT(−/−) and wild‐type (5‐HTT(+/+)) rats with a history of long access to cocaine or sucrose (control) self‐administration. We found that 5‐HTT(−/−) rats, compared with wild‐type control animals, self‐administered more cocaine, but not sucrose, under long‐access conditions. Ultrahigh‐resolution structural MRI subsequently revealed that, independent of sucrose or cocaine self‐administration, 5‐HTT(−/−) rats had a smaller amygdala. Moreover, we found an interaction between genotype and type of reward for dorsal raphe nucleus volume. The data point to an important but differential role of the amygdala and dorsal raphe nucleus in 5‐HTT genotype–dependent vulnerability to cocaine addiction.