Susceptibility to conditioned place preference induced by addictive drugs in mice of the C57BL/6 and DBA/2 inbred strains

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Abstract
In previous studies, we have demonstrated that mice of the inbred strain C57BL/6J (C57) are more susceptible to amphetamine-induced conditioned place preference (CPP) than DBA/2J (DBA) mice. Moreover, we also observed parallel strain differences for the locomotor-stimulant effects of the drug. However, other studies have reported either no difference or opposite strain differences for cocaine- and morphine-induced CPP as well as for the locomotor effects of these drugs, suggesting that amphetamine-related behavioral phenotypes might depend on a specific pharmacological action of the psychostimulant. This study was aimed at testing strain differences for cocaine- and morphine-related behavioral phenotypes in the same experimental protocol and conditions previously used for amphetamine. C57 and DBA mice were tested for CPP induced by cocaine (0, 5, 10, and 20 mg/kg) and morphine (0, 5, 7.5, and 10 mg/kg). Locomotor activity data were simultaneously obtained by measuring distance moved during all different CPP phases and unconditioned locomotor activity, behavioral sensitization and conditioned hyperactivity were measured together with CPP. (a) Either cocaine or morphine promoted significant CPP at lower doses in C57 than in DBA mice; (b) only drug-trained C57 mice showed a significant CPP compared with the control group; and (c) only C57 mice showed dose-dependent effects of cocaine on CPP. Moreover, there was no relationship between drug-induced CPP and locomotion. The results demonstrate that C57 and DBA mice differ in their sensitivity to cocaine- and morphine-induced CPP and suggest that the two strains differ in sensitivity to the positive incentive properties of drugs of abuse.