DIFFERENCES IN EXTRACELLULAR DOPAMINE CONCENTRATIONS IN THE NUCLEUS-ACCUMBENS DURING RESPONSE-DEPENDENT AND RESPONSE-INDEPENDENT COCAINE ADMINISTRATION IN THE RAT

Studies indicate that nucleus accumbens (NAcc) dopamine neurotransmiss ion is involved in the reinforcing and direct effects of cocaine. The present study was initiated to explore further the relationship of NAc c extracellular dopamine concentrations ([DA](e)) and cocaine self-adm inistration using a yoked littermate design, Tn the first experiment, one rat from each litter was trained to self-administer cocaine IV (SA ; 0.33 mg/inf) under a fixed ratio 2 schedule, while a second rat rece ived simultaneous infusions of cocaine yoked to the infusions of the S A (YC), NAcc [DA](e) and cocaine concentrations ([COC]) were assessed during the test sessions using in vivo microdialysis combined with mic robore HPLC procedures. [DA](e) and [COG] were significantly elevated in the SA and YC groups during the self-administration session; howeve r, [DA](e) were greater in the SA group compared to the YC group in th e first hour of the session, even though [COC] were not significantly different. On the following day, the rats previously allowed to self-a dminister cocaine were administered response-independent cocaine infus ions yoked to the infusion pattern from the previous day. [DA](e) were significantly elevated above baseline levels during the session but w ere significantly less than concentrations obtained when cocaine was s elf-administered by these subjects. [COG] during the sessions were not significantly different between the two days. Baseline [DA](e) were n ot significantly different between the SA and YC groups or between Day 1 and Day 2. Furthermore, there was no significant difference in the in vitro probe recovery between one and two days following probe impla ntation. These results suggest that the context in which cocaine was a dministered significantly altered the neurochemical response to equiva lent brain concentrations of cocaine. NAcc [DA](e) was significantly i ncreased when the delivery of cocaine infusions was contingent on the behavior of the rat, indicative of a role in the neural processes unde rlying cocaine reinforcement.