Relationships of open-field behaviour with anxiety in the elevated zero-maze test: Focus on freezing and grooming

The National Institutes of Health Genetically Heterogeneous Rat Stock (NIH-HS) is a unique tool for genetic studies of complex traits due to its high genetic heterogeneity and to its high level of genetic recombinants accumulated along many outbreeding generations. In the present study, 90 NIH-HS male rats were tested for anxiety/fearfulness in the elevated zero-maze and in the open-field test in order to investigate the associations among defensive responses from both tests and, in particular, those among open- field self-grooming and freezing. These associations were evaluated by means of a correlational-factorial approach and an analysis of differences between sub- groups displaying extreme scores in representative variables. The final factor analysis revealed a first factor with high loadings of all variables from the zero-maze (“Maze timidity/conflict” factor), and a second (independent) factor dominated by open-field crossings (-0.74), rearings (-0.62) and freezing (0.65), with lower loadings of open-field grooming (-0.39) and stretched attend postures, as well as of entries and time (loadings of -0.32 to -0.25) in the open sections of the zero-maze (“Open Behavior inhibition/ desinhibition” factor), suggesting that open-field self-grooming is a response associated to activity, in the present study, rather than to inhibition. Furthermore, the finding that grooming in the OF loaded negatively in a second factor supports a relationship between grooming and dearousal. Present results, thus, are in accordance with the usefulness of these tests for the purposes they are commonly employed and add new evidence supporting their concurrent validity, as indicated by the relationships observed among measures from both tests.