Repeated Cocaine Administration Alters Extracellular Glutamate in the Ventral Tegmental Area

Abstract: The present study determined if repeated cocaine injections alter the effect of cocaine on extracellular glutamate in the ventral tegmental area (VTA). All rats were treated with daily cocaine (15 mg/kg i.p. × 2 days, 30 mg/kg i.p. × 5 days) or saline for 7 days. At 21 days after discontinuing the daily injections, a dialysis probe was placed into the VTA and the extracellular levels of glutamate were estimated. A systemic injection of cocaine (15 mg/kg i.p.) elevated extracellular glutamate in the VTA of rats pretreated with daily cocaine but not in the daily saline-pretreated subjects. No significant change in glutamate was produced by a saline injection in either pretreatment group. In a group of rats pretreated with daily cocaine, the D₁ antagonist SCH-23390 (30 µ M ) was infused through the dialysis probe prior to the acute injections of saline and cocaine. SCH-23390 prevented the increase in extracellular glutamate associated with the acute administration of cocaine. Behavioral data were collected simultaneously with the measures of extracellular glutamate. The behavioral stimulant effect of cocaine was greater in cocaine-pretreated than saline-pretreated subjects, and the behavioral augmentation in cocaine-pretreated rats was partly blocked by SCH-23390. These data support the hypotheses that repeated cocaine administration produces an increase in the capacity of D₁ receptor stimulation to release glutamate in the VTA and that this mechanism partly mediates behavioral sensitization produced in rats treated with daily cocaine injections.