Fig. 6.

Mice subjected to social isolation exhibit increased vigilance and higher corticosterone reactivity. A, before separation into grouped or isolated housing conditions, innate vigilance was evaluated by measuring the time (in seconds) it took for each mouse to venture into the open field. Based on these measurements, mice were then separated into two cohorts (cohort A and cohort B) with a balanced distribution of vigilance; cohort A was then group-housed and cohort B was individually housed. B, following 9.5 wk of either group housing or isolation, behavioral testing was repeated. Overall, isolated animals displayed more vigilant behavior; that is, significantly longer times to enter an open-field compared with group-housed mice (P = 0.018 based on log-rank test). C, at 22 wk of age, the reactive stress response was assessed (n = 6 group-housed, n = 7 isolated) by measuring serum corticosterone levels before and following a 30-min mild restraint stressor; individual corticosterone levels over time (relative to baseline) are shown. Gray shading, the duration of restraint. D, no significant difference (N.S.) was found in baseline serum corticosterone levels. Error bars, SEM. E, isolated mice exhibit significantly increased corticosterone reactivity. The slope of the increase (reactivity) was determined by calculating the difference between the baseline and highest level for each animal divided by the time elapsed. Error bars, SEM (P = 0.022 based on the Mann-Whitney U test).