Decrease in basal dopamine levels in the nucleus accumbens shell during daily drug-seeking behaviour in rats.

Accumbal dopamine (DA) is generally accepted to participate in the neural mechanisms underlying drug dependence. Recently the involvement of accumbal DA in drug-seeking behaviour has gained more experimental attention. To study an involvement of accumbal DA in drug-seeking behaviour within and between daily self-administration behaviour, changes in extracellular DA concentration in the nucleus accumbens (NAc) shell were measured during the daily dynamics of intravenous heroin and cocaine self-administration. Groups of drug naive rats were allowed to intravenously self-administer heroin (30 microg/infusion) and cocaine (30 microg/infusion) during five consecutive daily 3 h sessions. Extracellular DA concentrations in the NAc were measured before and after a single 3 h session (acute) and before and after 5 consecutive 3 h sessions (repeated). Following acute and repeated heroin and cocaine self-administration the extracellular DA concentration in the NAc shell was increased by two-fold to three-fold over baseline. These changes in DA concentrations are thought to reflect a direct effect of heroin and cocaine on DA neurotransmission in the NAC shell. Measurement of basal DA concentrations before the self-administration sessions revealed that just before the scheduled 5th self-administration session the (absolute) basal DA levels in the NAc in heroin or cocaine self-administering animals were decreased by approximately halve, as compared to drug-naive animals. It is assumed that just before a scheduled next session the (daily) desire for the drug is high. This decrease in basal DA neurotransmission in the NAc shell may, therefore, reflect an involvement of accumbal DA in drug-seeking behaviour during daily self-administration behaviour. The results demonstrate that initiation of i.v. heroin and cocaine self-administration is linked with changes in extracellular levels of DA in the NAc shell. Moreover, the present data suggest that accumbal DA might be involved in processes underlying the motivational aspects involved in daily drug-seeking behaviour, and that neuroadaptive changes in the mesolimbic DA system due to repeated drug intake lead to an tonic decrease in overall DA activity in the NAc.