Fig. 4.

Impact of stress chronicity on subsequent fear learning reinforced by a novel aversive stimulus. Depicted: Schematic illustration of the timeline of the experiment and the figure panels in which data from various components of the experiment are displayed (panel A). Percent freezing during stress pretreatment (panels B & C, days 1–15), context preexposure (panel D, days 16–21), context test (panel E, day 23), and open field test (panel F, days 24–25). Rats received either acute (AS) or chronic (CS) exposure to 15 footshocks, or identical context exposure with no shock (No Stress; AN/CN). All groups were preexposed (30 min/day) to a novel environment for 6 consecutive days. Twenty-four hours after the termination of preexposure, all groups received a single startle stimulus in the preexposed context. All groups were tested for contextual fear learning 24 h later. Stressed rats exhibited enhanced contextual fear learning to the startle noise, but there was no difference between AS and CS (panel E). Additionally, stressed rats exhibited decreased exploratory behavior in the open field test as indicated by a decrease in distance travelled, but, again, there were no differences between AS and CS (panel F). Error bars denote mean ± SEM. *, ** denotes significance (p ≤ .05, p ≤ .01) compared between indicated groups (horizontal square brackets), compared between each Stress group and its respective No Stress control (horizontal line), or compared between a specific Stress group and its respective No Stress control (free-standing).