Molecular mechanisms underlying altered stress sensitivity during amphetamine withdrawal in male rats

Withdrawal from chronic amphetamine is associated with increased anxiety which leads to relapse. Dysregulation of serotonin is associated with elevated anxiety. Previous studies show a reduction in stress induced serotonin in the ventral hippocampus at 24 h withdrawal from chronic amphetamine in male rats, which is thought to be contributed by increased serotonin clearance by the organic cation transporter (OCT3). Chronic amphetamine also reduced serotonin in ventral hippocampus at 2 weeks withdrawal but conversely increased in central nucleus of amygdala (CeA) at 24 h and 2 weeks withdrawal. Therefore, we tested whether transporter expression in rats is altered in these brain regions over protracted withdrawal to account for the changes in serotonin levels. We also examined OCT3 and serotonin transporter (SERT) expression in additional brain regions that are associated with anxiety like behavior. We found that OCT3 and SERT expression were increased in CeA at 24h and 2 weeks withdrawal. Dorsomedial hypothalamus (DMH) and ventral hippocampus showed increased OCT3 expression at 24h withdrawal but not at 2 weeks withdrawal, while SERT expression remained unchanged at either time point. OCT3 expression remained unchanged in bed nucleus of stria terminalis (BNST), dorsal hippocampus and lateral septum. Such result indicate changes in serotonin transporters may cause imbalance in serotonin during amphetamine withdrawal. Next, we investigated the glucocorticoid receptor (GR) as a potential mechanism underlying increased OCT3 expression. This is because GR agonists increase OCT1 and OCT2 expression in other tissues, and amphetamine increases plasma levels of corticosterone, a GR agonist. We found that corticosterone increased OCT3 expression in the ventral hippocampus, which was not blocked by the GR antagonist mifepristone, suggesting a GR independent mechanism. Corticosterone also increased anxiety like behavior, while mifepristone decreased plasma corticosterone levels and increased SERT and GR expression in the ventral hippocampus. Neither treatments had any effect on the other brain regions studied, suggesting that ventral hippocampus is likely to be primary site of action of corticosterone and mifepristone. Overall, OCT3 could be an alternate target to treat negative affect associated with withdrawal from chronic amphetamine to prevent relapse.