Time course of changes in cocaine self-administration behavior in rats during immunization with the cocaine vaccine IPC-1010

Rationale: Following a 6-week immunization period consisting of three biwee kly injections of the cocaine vaccine IPC-1010. the reacquisition of cocain e self-administration behavior in rats was previously shown to be reduced i n a manner that was dependant on serum antibody level. The present studies were conducted to examine additional issues relevant to the clinical use of the vaccine. Objectives: One experiment was conducted to address the issue of whether exposure to cocaine during the immunization period would influe nce the ability of the vaccine to block cocaine self-administration. A seco nd experiment was conducted to determine if the reductions in drug-seeking behavior and drug intake by the vaccine were behaviorally specific. or if b ehavior maintained by a non-drug reinforcer would be similarly affected. Me thods: Identical second-order schedules of cocaine (1 mg/kg) or food pellet (45 mg) delivery were used in rats. In both studies, the time course of ch anges in behavior during the 6-week immunization period was examined in vac cine and alum-treated control rats following baseline and extinction condit ions. Results: The cocaine vaccine IPC-1010 induced average serum antibody levels of 0.07 mg/ml and significantly reduced self-administration behavior during the 2-week period following the third vaccine boost in a subgroup o f rats with serum antibody levels greater than the average value. Cocaine s elf-administration behavior at this time point significantly correlated wit h serum antibody level. IPC-1010 did not alter responding maintained by foo d throughout the immunization period although serum antibody levels reached a similar average of 0.06 mg/ml in this group of rats. Conclusions: These findings suggest that the reductions in drug-seeking behavior and drug inta ke after immunization with IPC-1010 did not result from a reduced ability o f the rats to respond on the lever. Furthermore, daily exposure to cocaine during the immunization period did not influence the ability of the vaccine to reduce cocaine self-administration behavior that emerged gradually over time. These findings also confirm the need for a sufficiently high antibod y level to blunt the reinforcing effects of cocaine.