Behavioral responses to repeated cocaine exposure in mice selectively bred for differential sensitivity to...

Mice from the 20th generation of three lines divergently selected for response to pentobarbital-induced sedation times [long-sedation time (LST), short sedation time (SST), and randomly bred control (RBC)] were used to study cocaine-induced behavioral sensitization. These lines showed variable degrees of locomotor activities in response to cocaine. At a low cocaine dose and long withdrawal period (10 mg/kg, twice a day for 5 days followed by a 14-day withdrawal), the LST mice showed tolerance development. In response to cocaine, the locomotor activities of the SST were not significantly different from the RBC group. At a higher dose and a shorter withdrawal period (20 mg/kg, daily for 7 days followed by a 3-day withdrawal), the SST mice showed behavioral sensitization similar to the RBC mice, but the LST mice did not develop sensitization. The different responses in locomotor activity induced by cocaine suggest that genetic factors may play a role in determining the magnitude of response to this drug. Dopamine (DA) levels did not differ significantly in either striatum (STR) or nucleus accumbens (NAC) for the cocaine-treated animals to their corresponding saline-treated controls. The affinity (Kd) of D2 in the NAC decreased significantly, without changes in density (Bmax), in the cocaine-treated SST and RBC mice. On the other hand, the density of D2 binding sites in the SST and the RBC mice in the STR was significantly increased in cocaine-treated groups without change in Kd. The LST mice did not show any changes in the Kd and Bmax in either the STR or the NAC. Taken together, these findings suggest that the changes in the Kd of D2 in the NAC and the Bmax of D2 in the STR may contribute to the differences in locomotor responses to cocaine exposure in these mouse lines.