Persistent sensitization of dopamine neurotransmission in ventral striatum (nucleus accumbens) produced by prior experience with (+)-amphetamine: a microdialysis study in freely moving rats

In humans the repeated use of amphetamine (AMPH) produces a hypersensitivity to the psychotogenic effects of AMPH that persists for months to years after the cessation of drug use. To explore the neurobiological basis of this phenomenon the long-term effects of dextroamphetamine ((+)-AMPH) on brain monoamines and behavior were studied in an animal model of AMPH psychosis. An escalating dose pretreatment regimen (from 1 to 10 mg/kg over 5 weeks) was used to mimic the pattern of drug use associated with the development of addiction and AMPH psychosis. The effect of pretreatment with AMPH on dopamine (DA) release in the ventral striatum (nucleus accumbens) was determined by measuring the extracellular concentrations of DA and DA metabolites using in vivo microdialysis, both before and after an AMPH challenge. The postmortem tissue concentrations of DA, serotonin and their metabolites were measured to determine if this treatment was neurotoxic. Escalating doses of (+)-AMPH were not neurotoxic, and 25–30 days after the cessation of drug treatment animals showed relatively normal levels of spontaneous motor activity across the day-night cycle. However, AMPH pretreatment produced robust behavioral sensitization. Animals showed a marked hypersensitivity to the motor stimulant effects of an AMPH challenge, even after 15–20 days of withdrawal. Most importantly, this hyperdopaminergic behavioral syndrome was accompanied by significantly elevated DA release in the ventral striatum. In contrast, AMPH pretreatment had no effect on the basal extracellular concentrations of DA. It is suggested that the sensitization produced by chronic AMPH use is due to enduring changes in the releasability of DA, and that this may represent an example of neural plasticity common to other forms of behavioral adaptation.