New index based on the physical separation of motion into three categories for characterizing the effect of cocaine...

Characterization of open-field behavior and locomotor activity is widely used to assess the influence of a drug on mouse or rat behavior. In this study, we developed an index for characterizing the behavior of cocaine-administered mice (C57BL/6, DBA/2, and BALB/c). Because a three-exponential-model exhibited the best fit to the obtained data among the different probability density functions, we divided each walking episode into three categories according to the duration of movement. We found a significant difference in decay variation of mean speed with time in the case of long walking duration. To clarify this difference quantitatively, we developed an index for the changes in locomotion control, based on a heuristic argument regarding the ratio of the coefficients of the drag term obtained by the biphasic motion-equation model. The index had a significant dose-related effect in each strain and a significant strain effect in high-concentration drug. Therefore, it would thus be useful for examining the effect of the drug on locomotor activity in mice. Moreover, evaluating other characters suggested previously, the proposed index had good advantage to differentiate the dose-related response in the three species of inbred mice.