Enhancement of Delayed Release of Dopamine in the Amygdala Induced by Conditioned Fear Stress in Methamphetamine-sensitized Rats

Behavior during conditioned fear stress, a form of psychological stress, and the release of dopamine in the amygdala were measured over time using methamphetamine-sensitized rats, which are considered to be a model of hypersensitivity and vulnerability to emotional stress associated with stimulant-induced psychosis and schizophrenia. Dopamine release in the amygdala showed a delayed increase following completion of freezing behavior induced by conditioned fear stress regardless of the presence or absence of methamphetamine-sensitization. Since methamphetamine treatment did not lower the basal level of dopamine in the amygdala, under the conditions of this study, methamphetamine was presumed not to show neurotoxicity. On the other hand, basal dopamine levels after 15 h of repeated electric foot shock were about 40% lower than those in the control group (p<0.0002). In addition, dopamine release following conditioned fear stress in animals repeatedly treated with methamphetamine increased significantly from 40 to 100 min after conditioned fear stress while the duration of freezing behavior or latency of the appearance of grooming were not different from those in the control group. The above results suggested that delayed dopamine release in the amygdala is a phenomenon strongly associated with the emotional context of conditioned fear stress, and hypersensitivity and vulnerability to stress are at least partially involved with the overreaction to stress.