Sensitization of fear learning to mild unconditional stimuli in male and female rats.

Stress-enhanced fear learning (SEFL) refers to the long-lasting nonassociative sensitization produced by intense stress (e.g., repeated and unpredictable footshock) that results in increased fear learning to a mild conditioning regimen (e.g., one shock). SEFL experiments suggest that one component of posttraumatic behavior is inappropriately strong fear conditioning occurring to relatively mild stressors. Past reports of SEFL have used the same intensity (1 mA) of footshock to cause both the sensitization and conditioning of new fear. SEFL would be a particularly problematic component of posttrauma behavior if intense stress results in substantial fear conditioning under conditions that would not normally support conditioning. Therefore, we determined if SEFL occurred when the conditioning shock was substantially milder than the SEFL-inducing shock. The results indicate that exposure to a sensitizing regimen of shock can convert a mild footshock that normally does not support measurable levels of fear conditioning into one that causes substantial learned fear. Moreover, as the intensity of single footshock increases, so does the capacity of the prior stressor to contribute to the sensitization of fear responses. Consistent with prior studies, males acquired and retained a greater level of fear conditioning than female rats, however the level of sensitization did not differ between sexes.