Rationale:

Hyperactivity and hypoactivity are symptoms of many neuropsychiatric disorders. In addition, many advanced behavioral assays are locomotion-dependent and cannot be performed on animals with severely impaired locomotor activity (LMA). Therefore LMA of genetically modified or pharmacologically treated mice is often assessed prior to advanced testing to assess any effects of locomotion on the results of other behavioral assays. The reaction to a novel environment and anxiety parameters are also assessed in this assay. The percent path length and the time spent in the center of the arena are often used as measures of anxiety-like behavior in mice. Generally, mice avoid the center of the open field apparatus, the most exposed part of the arena. An increase in activity or time spent in the center of the open field is interpreted as decreased anxiety-like behavior. In our open field paradigm, the mouse movements are followed by a video-racking system in a square shaped apparatus during a one-hour period. The locomotor activity in 5 min bins, the total path length, the percent path length and the time in the center of the arena are determined.

Methods:

Group housed mice aged 10 weeks were used for this experiment, maintained on a light:dark cycle (6 am: 8 pm) with food and water available ad libitum. The mice were acclimated to the testing room for at least 30 minutes. The assay was performed in a custom-made open field apparatus. Each chamber is a 50 by 50 cm square. The experiment was recorded and tracked by a video tracking system (Videotrack,Viewpoint).

For each mouse, the total path length and the path length in 5 min bins were determined for a 30 min period, as measures of locomotor activity. The path length in 5 min bins was analyzed by a two-way ANOVA with genotype as a between subjects factor and time as a within subjects (repeated measures) factor using SPSS software. All other measurements were analyzed using an unpaired t-test (GraphPad Prism).

The time and the path length in the center of the open field were determined as measures of anxiety-like behavior. The center of the open field was defined as a 13.5 x 13.5 cm square in the geometric center of the arena. The percentage of the animals' path in the center of the field was calculated as

Path length in the center X 100%

Total path length