Paradoxical increase of exploratory behavior in the elevated plus-maze by rats exposed to two kinds of aversive stimuli.

Albino rats were submitted to a 24-h period of social isolation (individual housing) combined with 0, 1, 2 or 3 twenty-four-hour periods of exposure to different vivaria (novelty) and tested in the elevated plus-maze. Results, reported as mean +/- SEM for N = 12, show that the time (in seconds) spent in the open arms by rats exposed to novelty for 0, 1, 2 and 3 days was 28.3 +/- 4.4, 31.6 +/- 3.2, 29.1 +/- 3.5 and 25.0 +/- 3.3, respectively, when grouped in the same vivarium; 29.6 +/- 2.7, 7.6 +/- 2.1, 9.6 +/- 4.4 and 28.5 +/- 3.7 when grouped in different vivaria; 2.9 +/- 1.1, 1.8 +/- 1.0, 2.7 +/- 1.1 and 0 +/- 0 when isolated in the same vivarium, and 2.6 +/- 1.1, 31.5 +/- 8.2, 24.8 +/- 4.2 and 0 +/- 0 when isolated in different vivaria. The number of entries into the open and closed arms followed a similar trend. This indicates that, separately, both exposure to novelty and isolation are aversive manipulations. Paradoxically, when novelty was combined with a concomitant 24-h period of social isolation prior to testing, the decrease in exploratory behavior caused by either of the two aversive manipulations alone was reverted. These results are indicative that less intense anxiety triggers mechanisms mediating less energetic behavior such as freezing, while higher levels trigger mechanisms mediating more vigorous action, such as flight/fight behavior, since the combination of two aversive situations resulted in more exploratory behavior than with either alone. They are also suggestive of habituation to the effects of novelty, since exposure to it for 3 days produced exploratory behavior similar to that of controls.