Effects of under- and overcrowding on exploratory behavior in the elevated plus-maze.

The present work investigated whether the number of rats housed in a cage affects exploration of an elevated plus-maze. Male Wistar-derived rats were kept 1, 2, 3, 4, 6, 8, 12, 16, or 24 to same size cages either for 1 or 14 days and tested in the elevated plus-maze. Rats kept 6 to a cage were arbitrarily considered controls because this is the housing condition adopted in many laboratories, ours included. In comparison to controls, 1-day housed rats kept 1, 2, 16, and 24 to a cage decreased the percentage of entries into the open arms. Similar decreases were also found in the time spent in the open arms, the only exception being the group with rats kept 16 to a cage which failed to show significant differences from the control group. Fourteen-day housed rats kept 1, 2, 16, or 24 to a cage decreased the percentage of entries and time spent in the open arms. We found plus-maze exploration to be similar in groups in which rats were kept from 4 to 12 to a cage. The present data indicate that anxiogenic effects resulting from under- and overcrowding should be taken into consideration in behavioral studies.