Two Types of Strain Differences in Open-Field Behavior in Rats and their Mapping

Open-field behavior of three treatments for three strains of rats (SD, Long-Evans, Fischer-344) was plotted on a map by component scores that were derived from principal component analysis. The treatments consisted of electric shock, extensive stroking, or testing after habituation to the field. Five measures were used: ambulation, penetration into the inner square, rearing, defecation, and urination. There was a clear strain difference on the basis of a raw open-field test score. This was termed a raw-score strain difference. A treatment-effect strain difference, however, which is obtained by comparing treatment and control conditions, was not clear. Thus, the behavior of each treatment condition cohered clearly with each other on the map, regardless of raw-score strain differences. This indicates that the nature of emotional behavior caused by the present treatments is very similar among strains despite the clear raw-score strain difference.