However, the lower CORT concentrations 30 min after the onset of stress in the

repeatedly stressed group compared to the CORT concentrations in the acutely stressed

group suggests a sensitization in the negative feedback regulation of CORT.

 Decreased thymus mass and/or increased adrenal mass has been shown to result

from chronic stress regimens involving physical and/or psychological stressors (for

examples, see Blanchard et al., 1998; Bryant et al., 1991; Simpkiss and Devine, 2003) as

well as from major depression in victims of suicide (Szigethy et al., 1994). Given that

two of the three groups that were stressed daily (C/A-10 and C/A-30) exhibited thymus

involution while one (C/NA) did not, it cannot be concluded that the daily stress regimen

consistently caused a decrease in thymus mass. These findings, along with adrenal mass

equivalency across groups regardless of number of stress exposures, suggest that the

chronic social defeat stress regimen of six daily exposures may not have been adequate to

consistently alter glandular masses. A longer or unpredictable social defeat stress

regimen could possibly generate consistent thymus involution and adrenal hypertrophy.

Perhaps an additional stressor for the intruders, such as isolation housing to ensure more

timid behavior (Kabbaj et al., 2000), could contribute to the effectiveness of the regimen.

Also, using a restraint stress harness instead of the protective wire mesh cage could

elevate the efficiency of the regimen by producing an inescapable condition for the

intruders .

 Intermittent Social Defeat Stress

 The equivalent progressive increases in freezing behavior and the equivalent

progressive decreases in exploratory locomotion in the intermittent stress regimen

indicate that the repeatedly stressed groups underwent comparable stress-induced

behavioral changes. However, the escalation of freezing behavior and the decline of