Some critical determinants of the behaviour of rats in the elevated plus-maze.

The effects of daytime testing periods, repeated test exposures, and level of illumination were tested on the behavior of rats in an elevated plus-maze consisting of two open and two enclosed arms. Rats made significantly more entries into the open arms and spent significantly more time in the open arms when testing was carried out between 8 h and 12 h than when performed between 14 h and 17 h. Repeated exposure to the test apparatus tended to reduce time spent by rats in the open arms, number of entries into the open arms and total locomotor activity. Finally, it was found that an increase of the level of illuminance was followed by a decrease of all behavioural parameters. Since conflicting results have been reported for drug treatments evaluated in the elevated plus-maze (e.g., for compounds acting at the 5-HT1A receptor), the present results, based on the experimental conditions used, provide one possible explanation for these discrepancies.