Repeated stressful experiences differently affect the time-dependent responses of the mesolimbic dopamine system to the stressor.

The increase in mesolimbic dopamine (DA) release observed during the first 40 min of 120 min restraint in naive rats is not evident in repeatedly stressed animals (daily 60 min restraint, for 5 days). However, repeatedly stressed rats show a significant decrease in DA release from 80 min of restraint onwards which is not observable in naive rats. These results indicate that repeated stressful experiences do not produce habituation but alter the response of mesolimbic DA system to the stressor. Moreover, they point to a possible neuronal mechanism underlying stress-induced depression.