Genetic analysis of sensitization and tolerance to cocaine

The present study investigated the effects of acute and repeated administration of cocaine (1.0-56.0 mg/kg) on locomotor activity in the genetically distinct DBA/2J and C57BL/6J inbred strains of mice. In addition, quantitative trait loci analysis of the effects of acute and repeated cocaine in 16 BXD recombinant inbred strains was used to provisionally detect and map minor gene loci which associate with cocaine responsiveness. Whereas locomotor activity was elevated maximally in both strains by 32 mg/kg of cocaine, DBA/2J mice were stimulated to a much greater extent than C57BL/6J mice. The stimulant effects of cocaine were diminished to control levels in DBA/2J mice after repeated daily injections, whereas cocaine-induced locomotion remained consistent in C57BL/6J mice throughout the 7-day testing period. Emergence of stereotyped behavior with repeated daily injections of 32 mg/kg of cocaine was observed in DBA/2J but not C57BL/6J mice. No differences in brain cocaine levels were found between the DBA/2J and C57BL/6J strains after acute or repeated injections. Quantitative trait loci analysis indicated significant associations of differences in cocaine responsiveness with marker loci on several chromosomes in the BXD recombinant inbred series. Those marker loci associated with the acute cocaine response were in most cases different from those markers associated with long-term responses. The current results demonstrate that genotype-dependent variation exists in behavioral responsiveness to cocaine in mice and suggest that the acute and long-term responses to cocaine may be under the control of separate sets of genes.